Your Subaru’s Boxer engine is a finely tuned machine, but it has a critical weakness: oil blow-by. This common issue pushes oil into the combustion chamber where it doesn't belong, seriously increasing the risk of catastrophic engine knock. A quality Subaru Air Oil Separator is the most effective insurance policy against this threat. By installing an AOS, you are actively preventing a root cause of many engine failures. It’s a relatively small investment that provides immense peace of mind by safeguarding your pistons, rods, and bearings from expensive damage.
Key Takeaways
- An AOS is a protective necessity, not just an upgrade: It prevents oil vapor from contaminating your intake, which maintains your fuel's octane rating and stops destructive engine knock. This is especially critical for any modified or turbocharged Subaru.
- Recognize the warning signs of a failing system: Pay attention to increased oil consumption, blue smoke from the exhaust, and sluggish performance. Identifying these symptoms early is key to preventing more serious and costly engine damage.
- Invest in a maintenance-free, closed-loop system: A high-quality AOS automatically separates and returns oil to the engine, making it a "set it and forget it" solution. This is a far more effective and reliable choice than a basic catch can that requires constant manual draining.
What is a Subaru Air Oil Separator?
Think of an Air Oil Separator (AOS) as your engine's personal bodyguard. In any engine, but especially in high-performance Subaru Boxers, pressure builds up in the crankcase. This pressure, a mix of air, unburnt fuel, and oil vapor, has to go somewhere. A standard PCV (Positive Crankcase Ventilation) system vents these gases back into your intake manifold to be burned off. The problem? That oil vapor comes along for the ride. When oil contaminates your air-fuel mixture, it lowers your fuel's octane rating and can lead to a host of performance issues, from reduced power to serious engine knock.
An AOS is an elegant solution that intercepts these vapors before they reach your intake. It’s designed to separate the oil from the air, sending clean air back to the intake and draining the collected oil safely back into your engine's oil pan. Unlike a simple catch can that just collects the gunk and needs to be emptied regularly, a quality Subaru Air Oil Separator is a true closed-loop, maintenance-free system. It’s a must-have for any modified Subaru, providing a critical layer of protection whether you're on the track, trail, or just enjoying a spirited daily drive. By keeping your intake clean, an AOS ensures your engine performs consistently and reliably, protecting your investment in the long run.
How Does an Air Oil Separator Actually Work?
The magic of an AOS happens inside its canister. As the crankcase vapors are routed into the unit, they pass through a series of baffles or a centrifugal chamber. This process forces the heavier oil and fuel droplets to separate from the lighter air. Gravity takes over, and the collected oil drains down to the bottom of the AOS. From there, it’s returned directly to the oil pan, keeping your oil levels consistent. Meanwhile, the now-clean air is sent back into the engine’s intake tract to be burned as intended. This continuous, self-contained cycle ensures your engine is only breathing in fresh air, not a performance-robbing oil mist. It’s a simple yet highly effective process that makes the AOS one of the most important supporting performance parts you can install.
Separation Methods: Swirl Pots, Filters, and Heating
Not all Air Oil Separators are created equal; the technology inside is what truly sets them apart. The most effective units employ a multi-stage process to scrub oil from the crankcase vapor. Many high-quality systems utilize swirl pot technology, which uses centrifugal force to spin the incoming air. This action slings the heavier oil and fuel droplets against the canister's walls, allowing them to condense and drain away. This is often paired with a series of internal baffles or filters that act as a physical barrier, catching any remaining contaminants. The most sophisticated performance parts also integrate a heating mechanism, typically by circulating engine coolant through the AOS. This keeps the unit warm, preventing water vapor from condensing and mixing with the oil to form sludge. Instead, water and fuel are vaporized and sent back into the intake, ensuring only clean oil returns to your engine pan.
How an AOS Protects Your Engine from Damage
By keeping oil out of your intake, an AOS directly prevents one of the biggest threats to a tuned engine: detonation, also known as engine knock. When oil vapor mixes with the air and fuel entering your combustion chamber, it effectively lowers the octane rating of your fuel. This makes the mixture more prone to igniting prematurely under pressure, creating a destructive shockwave inside the cylinder. This is especially dangerous in turbocharged engines like those in the WRX and STI. An AOS ensures a clean, potent air-fuel charge, which allows your engine to run safely at its peak potential. It protects your pistons, rods, and bearings from the catastrophic damage that detonation can cause. In short, it’s not just a performance part—it’s essential insurance for your engine’s health and longevity.
Why Turbocharged and Direct Injection Engines Need an AOS
Turbocharged and direct-injection engines, the powerhouses behind models like the Subaru WRX and STI, operate under intense heat and pressure. This demanding environment makes them especially vulnerable to the effects of oil blow-by. When oil vapor finds its way into your intake and mixes with the air-fuel charge, it effectively lowers your fuel's octane rating. A lower octane rating reduces the fuel's ability to resist detonation, or "engine knock," which is a major threat in any high-performance engine. An AOS acts as the ultimate defense, preventing oil vapor from ever contaminating your intake. This crucial separation maintains your fuel's integrity, allowing your engine to safely handle the demands of high-performance driving and custom tuning without the risk of destructive knock.
The Unique Needs of the Subaru Boxer Engine
The Subaru Boxer engine’s horizontal layout is fantastic for creating a low center of gravity and excellent balance, but this design also makes it more prone to oil blow-by. Because of its flat configuration, pressure can easily build in the crankcase, forcing oil vapor into places it doesn't belong. An AOS acts as a personal bodyguard for your engine, intercepting these vapors before they can cause any trouble. By ensuring only clean, dry air enters the combustion chamber, an AOS helps create a potent and reliable air-fuel charge. This allows your engine to operate at its full potential, whether you're on the track or navigating a trail with an overlanding build. Most importantly, it protects critical internal components like your pistons, rods, and bearings from the catastrophic damage that detonation can cause.
Signs Your Subaru AOS is Failing
A failing Air Oil Separator doesn't usually announce itself with a loud bang. Instead, it sends subtle signals that can be easy to miss if you aren't paying attention. This component works quietly in the background, but when it starts to fail, the consequences can be significant for your engine's health and performance. Ignoring the early signs allows oil to contaminate your intake system, which can lead to reduced performance and, eventually, serious engine damage. Catching the problem early is key to protecting your investment and keeping your Subaru running at its peak, whether you're on the street, track, or trail. Think of it as preventative care for your engine's heart. If you notice any of the following symptoms, it’s time to take a closer look at your AOS. These signs can range from visible smoke and higher oil consumption to a gradual loss of power, and identifying them is the first step toward a solution that keeps your engine healthy and your performance consistent. Don't wait for a check engine light; being proactive here can save you from a much larger repair bill later on.
Excess Smoke and Oil in Your Intake System
One of the most telling signs of a failing AOS is finding oil in your intake tract, intercooler, or turbo inlet. The entire job of the AOS is to prevent this from happening. When oil vapor gets past the separator and into the intake, it lowers the octane rating of your fuel-air mixture. This can cause engine "knock," also known as detonation, which can severely damage your pistons and reduce power. You might also see puffs of blue-ish smoke from your exhaust, especially during hard acceleration, as this oil is burned off in the combustion chamber. This isn't just messy; it's a direct symptom of a system that's no longer protecting your engine's most critical performance parts.
Poor Engine Performance and High Oil Consumption
Is your Subaru feeling a bit sluggish or less responsive than it used to be? A failing AOS could be the culprit. When oil coats the inside of your intercooler, it reduces its ability to cool the air entering your engine, leading to a drop in performance. You may also find yourself adding oil between changes more often than usual. This increased oil consumption happens because the oil that should be returned to the crankcase is instead being sent into the intake and burned away. Neglecting these symptoms can lead to more significant issues down the road, making a high-quality AOS a critical component for maintaining engine health and getting the most out of your engine tuning services.
Other Warning Signs: Sludge, Squeals, and Check Engine Lights
Beyond the more common symptoms, your Subaru might give you other clues that its AOS is struggling. A milky, sludge-like substance under your oil cap or in the PCV hoses is a major red flag. This happens when oil and moisture mix and aren't properly separated, creating a gunk that can clog the entire system. You might also hear a high-pitched squealing or whistling noise at idle, which often points to a vacuum leak caused by a failing valve within the AOS. Finally, while a check engine light can mean many things, it can be triggered by AOS failure due to issues like lean air-fuel mixtures or engine misfires. These warnings are your engine’s cry for help, signaling it's time to invest in reliable performance parts before minor issues become major engine damage.
How to Test Your AOS with a Manometer
If you suspect your AOS is failing but want a definitive answer, a diagnostic test is your best bet. The most reliable way to check if your AOS is working correctly is to use a tool called a manometer to measure your engine's crankcase vacuum. This simple test involves connecting the manometer to your oil dipstick tube and reading the pressure at idle. A healthy PCV system, managed by the AOS, should pull a slight vacuum. If you see positive pressure, it’s a clear sign that the system isn't functioning properly and crankcase vapors aren't being evacuated effectively. This is a straightforward test you can do at home or ask your trusted mechanic to perform.
Benefits of an Upgraded AOS
Swapping your stock Air Oil Separator for a high-performance model is one of the smartest investments you can make for your Subaru’s engine. This isn't just about replacing a part; it's a proactive upgrade that delivers tangible benefits in engine protection, efficiency, and overall performance. A quality AOS acts as a guardian for your engine, ensuring that only clean air enters your intake system. This is especially critical for turbocharged engines, where maintaining a clean air charge is essential for consistent power and long-term health. Whether you’re building a daily driver, a track weapon, or an off-road rig, an upgraded AOS addresses the inherent weaknesses of the factory PCV system. By effectively removing oil and other contaminants from the crankcase vapor, it prevents a cascade of issues that can rob your engine of power and eventually lead to costly damage.
Extend Engine Life and Reduce Carbon Buildup
The primary job of an upgraded AOS is to protect your engine from the inside out. Your Subaru’s engine naturally produces crankcase pressure that contains oil vapor. A high-performance AOS intercepts this vapor, separating the oil and returning it to the crankcase while allowing clean air to pass into the intake manifold. This process is crucial for preventing carbon buildup on your intake valves and pistons. Over time, that buildup can restrict airflow and create hot spots in the combustion chamber, leading to detonation, or "engine knock," which can cause catastrophic engine failure. By installing an upgraded AOS, you ensure your engine breathes cleaner air, which helps maintain its power and significantly extends its lifespan.
Eliminate Oil Blow-By for a More Efficient Intercooler
Oil blow-by is a common issue in performance engines, where oil vapor coats the inside of your intake tract and intercooler. This oily residue acts as an insulator, drastically reducing your intercooler's ability to cool the charged air before it enters the engine. Hotter air is less dense, which means less power and a higher risk of detonation. An upgraded AOS stops this process at the source. By capturing that oil before it can contaminate your system, it keeps your intercooler working at peak efficiency. This ensures you get the consistent, reliable power you expect, making your engine tuning services more effective and your vehicle more responsive.
Better Performance on the Track and Trail
When you’re pushing your Subaru on the track or navigating a tough trail, your engine is under extreme stress. High G-forces, aggressive cornering, and steep inclines cause oil to slosh around, overwhelming the factory PCV system and leading to increased oil consumption. This is exactly when you need your engine to perform flawlessly. A purpose-built AOS is designed to handle these demanding conditions, ensuring a steady supply of oil-free air to the engine, no matter the situation. This upgrade provides the reliability and consistent performance needed for lap-after-lap track days and is a vital component for any serious overlanding solutions build where engine dependability is non-negotiable.
Improved Oil Efficiency
An upgraded AOS contributes directly to better oil efficiency by creating a closed-loop system for your engine's oil. Instead of allowing oil vapor to escape through the crankcase ventilation system and get burned up in the combustion chamber, a high-quality AOS captures it. The separated oil is then cleanly returned to the oil pan, where it belongs. This process significantly reduces overall oil consumption, meaning you'll spend less time worrying about topping off your oil between changes. For performance-tuned engines, especially those in the Subaru STI or WRX, maintaining consistent oil levels is critical for lubrication and protection under high stress. By ensuring oil stays in the system, an AOS helps preserve your engine's health and longevity.
Air Oil Separator vs. Catch Can
When looking for ways to manage oil blow-by, you'll often see two options discussed: the Air Oil Separator (AOS) and the catch can. While both are designed to keep oil out of your intake, they are fundamentally different. A catch can is a simple, passive device—essentially a small container installed in the PCV line that "catches" oil vapor and condensation. It works, but it requires you to manually drain it on a regular basis. If you forget, or if it fills up during a long drive, it becomes completely ineffective, allowing oil to flow directly into your intake system. It’s a basic fix for a complex problem.
An AOS, on the other hand, is a comprehensive, active solution. Unlike a catch can that just stores the gunk, a quality Subaru Air Oil Separator is a true closed-loop, maintenance-free system. It not only separates the oil vapor from the crankcase gases but also automatically drains the collected oil back into the engine's oil pan. This means you never have to worry about emptying a container or the system failing because it's full. It’s a "set it and forget it" upgrade that provides continuous protection, making it a far more reliable and effective choice for any Subaru owner who values engine health and consistent performance.
Why an AOS is a Safer Choice for Track Driving
On the track, your engine is pushed to its absolute limit. The high G-forces from aggressive cornering and braking cause oil to slosh around violently in the crankcase, overwhelming the factory PCV system. Under these conditions, a simple catch can can fill up in just a few laps. Once full, it’s as good as not being there, allowing oil to flood the intake, coat the intercooler, and contaminate the air-fuel mixture. This contamination lowers your fuel's octane rating and dramatically increases the risk of detonation—the single most destructive force for a performance engine. An AOS is built to handle these extreme demands without fail.
This is where an AOS becomes a protective necessity, not just an upgrade. A purpose-built Air Oil Separator is engineered to function flawlessly even under the most demanding track conditions, ensuring a constant supply of clean, oil-free air to your engine. By preventing detonation, it protects your pistons, rods, and bearings from catastrophic failure when you need reliability the most. For anyone investing in engine tuning services or other high-end performance parts, a high-quality AOS is the only choice. It’s the essential safeguard that protects your investment and allows you to push your car with confidence, lap after lap.
How to Choose the Right AOS for Your Subaru
With several excellent Air Oil Separators on the market, picking the right one comes down to your specific Subaru model, your driving style, and the features that matter most to you. Each brand brings something different to the table, whether it's a track-tested design or specific features for daily driving. Understanding these differences is key to making an informed decision that will protect your engine for years to come. The goal is to find a system that not only fits your vehicle perfectly but also aligns with how you use it, from daily commutes to demanding track days or off-road adventures.
When you’re ready to upgrade, consider the materials, design, and reputation of the manufacturer. A well-engineered AOS is a critical investment in your Subaru’s health and performance, preventing costly issues down the road. Let’s break down some of the top contenders to help you find the perfect match for your build.
Spotlight: Crawford Performance AOS Systems
As the original and proven solution for Subaru engines, the Crawford Performance AOS systems are engineered with one primary goal: to protect your engine and extend its life. These systems are meticulously designed to prevent oil blow-by from contaminating your intake and intercooler. This is essential for maintaining consistent performance and preventing detonation, a phenomenon that can cause serious engine damage. By effectively separating oil vapor from the crankcase gases, our AOS ensures only clean air is recirculated, keeping your engine running strong whether you're on the street or the track.
Track-Tested and Proven by the Pros
The ultimate test for any component is how it holds up under extreme competition, and this is where our AOS truly shines. These separators aren't just designed in a lab; they have been battle-hardened in the world of professional motorsports, used in race-winning vehicles, including those driven by the legendary Ken Block. When a vehicle is subjected to high G-forces from aggressive cornering or navigating steep inclines on a trail, the oil in the engine sloshes violently, easily overwhelming the factory PCV system. Our AOS is engineered specifically for these demanding conditions, acting as a steadfast guardian for your engine by ensuring a consistent supply of clean, oil-free air to the intake. This professional-grade reliability is at the core of all our performance parts, giving you the confidence that your engine is protected by the same technology trusted by the pros.
PERRIN vs. IAG: A Head-to-Head Comparison
When exploring other options, you'll likely come across PERRIN and IAG Performance, both of which are respected names in the Subaru community. The PERRIN Air Oil Separator is designed to reduce carbon buildup and minimize detonation risk, featuring a heated base to prevent oil sludge from forming in colder conditions. IAG Performance also offers a wide variety of AOS kits tailored to different Subaru models, including a "Universal Drag Catch Can" for dedicated racing applications. Both brands focus on engine protection, but their designs and features cater to slightly different priorities and use cases.
Brand-Specific Features and Parts
The main differences between these top brands often come down to design philosophy and specific features. The Crawford Performance AOS is engineered with a singular focus on engine protection and longevity, using a design that has been track-tested for years to ensure only clean air is recirculated. PERRIN's AOS, on the other hand, includes a heated base, which is a great feature for drivers in colder climates as it helps prevent oil sludge from forming. IAG Performance stands out by offering a wide variety of kits tailored to different Subaru models and even provides a "Universal Drag Catch Can" for dedicated racing builds. Your choice depends on your specific needs: the proven, all-around protection of Crawford, the cold-weather consideration of PERRIN, or the specialized application options from IAG.
Warranties and Legal Considerations
Before you make a final decision, it's smart to look at the warranty and legal status of the part. A solid warranty shows that the manufacturer stands behind its product. For example, Crawford Performance parts come with a standard 12-month or 12,000-mile performance warranty, giving you confidence in your investment. It's also crucial to remember that many high-performance parts are intended for "off-road use only." This means they may not be legal for street use in areas with strict emissions regulations. Always check the product description and your local laws before purchasing and installing an AOS to ensure you stay compliant. This simple step can save you from potential headaches with inspections or tickets down the line.
Key Features to Look For and How to Check Compatibility
When making your final choice, focus on two things: key features and model compatibility. Do you need a heated unit for a cold climate, or is a track-proven, maintenance-free system like Crawford’s a better fit? Most importantly, you must confirm the AOS is designed for your specific Subaru. An AOS for a WRX won't necessarily fit a Crosstrek. Always check the product specifications to ensure it matches your vehicle’s model and year range. This guarantees proper installation, optimal function, and the engine protection you’re looking for.
Confirming Model and System Compatibility
Getting the right AOS for your Subaru isn't a guessing game; it's about precision. The mounting points, hose routing, and engine bay layout can vary significantly between models and even across different generations of the same model. For instance, an AOS engineered for the turbocharged engine of a Subaru STI will have completely different fitment requirements than one designed for a naturally aspirated Subaru Crosstrek. Before you add any performance parts to your cart, make it a habit to verify the product specifications against your vehicle’s exact model and year. This simple step ensures a smooth installation and guarantees the AOS will function as intended, providing the critical engine protection you're investing in.
Installing Your AOS: What to Expect and What You'll Pay
So, you’re sold on the benefits of an AOS and ready to get one installed. The next logical questions are: what does the installation involve, how long will it take, and what’s the damage to your wallet? Getting this performance part on your Subaru is usually a straightforward affair, but the details matter. Whether you're a seasoned DIYer ready to spend a Saturday in the garage or you prefer to hand the keys to a trusted mechanic, understanding the process and costs will help you make the right call. This section breaks down the installation timeline, the price range for quality units, and the pros and cons of installing it yourself versus having a professional handle it.
Understanding the Installation Process and Timeline
Installing an Air Oil Separator is a project you can likely tackle in an afternoon. Most installations take around three hours, making it a manageable weekend task for anyone comfortable working on their car. The process typically involves mounting the AOS unit securely in the engine bay and then routing hoses to connect it to your engine's crankcase ventilation system. While the exact steps vary depending on your specific Subaru model and the AOS kit you choose, the goal is always the same: to intercept the oily vapor before it reaches your intake and causes performance issues.
Installation Prerequisites and Required Tools
Before you get started, let's cover the basics for a smooth installation. The most important rule is to work on a completely cool engine—this is non-negotiable for your safety. Once you're ready, take the time to read through the installation guide that came with your specific AOS kit. While the general idea is the same, the mounting points and hose routing can vary quite a bit between different models, like the setup for a Subaru STI versus a Forester. Having a clear understanding of the steps before you start is the best way to avoid any surprises and ensure a stress-free afternoon in the garage.
You don’t need a professional-grade workshop to get this done, just a solid set of basic hand tools. Make sure you have a 3/8" ratchet with 10mm and 12mm sockets, plus a few extensions to help you reach bolts tucked away in the engine bay. You’ll also need a set of Phillips and flathead screwdrivers, a quality pair of hose cutters for clean, precise cuts, and some zip ties to keep everything tidy. While not strictly required for all kits, having a torque wrench on hand is always a good idea to ensure every bolt is tightened to the correct specification, giving you extra peace of mind that the job was done right.
AOS Cost Breakdown by Brand and Quality
The price for an Air Oil Separator can vary quite a bit, so it’s important to know what you’re paying for. You can find basic units for under $150, but high-performance systems from reputable brands typically range from $400 to $500. For example, our own Crawford Performance AOS kits are engineered for maximum efficiency and durability. The price difference often comes down to design complexity, material quality, and the brand’s engineering and testing. A well-made AOS is an investment in your engine’s health, so choosing a quality unit is key to getting the protection you need.
Should You DIY or Hire a Pro for Installation?
Deciding whether to install the AOS yourself or hire a pro comes down to your comfort level and toolset. If you have basic tools and enjoy working on your car, this is a project you can definitely do yourself. However, if you’re not confident, there’s no shame in leaving it to the experts. Think of it this way: a $400 part and a few hours of labor can prevent a potential multi-thousand-dollar engine rebuild down the road. For that kind of peace of mind, professional engine tuning services are a smart investment to ensure everything is installed and functioning perfectly.
Where to Buy Your Subaru AOS
Once you’ve decided to upgrade, the next step is finding the right place to purchase your Air Oil Separator. You want to buy from a source that not only offers a quality product but also provides solid support and stands behind their parts. Investing in a critical component like an AOS from a reputable seller ensures your engine gets the protection it deserves, giving you peace of mind whether you're on a daily drive or pushing your car to its limits.
The Best Places to Buy Your AOS Online
When shopping for an AOS, it’s best to start with specialists who live and breathe Subaru performance. As the creators of the original Subaru AOS, we at Crawford Performance offer a range of proven Air Oil Separators designed for nearly every model from 2002 to the present. Other well-known brands like IAG Performance and PERRIN also offer their own systems through various online retailers. While marketplaces like eBay can show you a wide selection of parts, sticking with a dedicated performance shop or the manufacturer directly often gives you better access to expert advice and support for your specific build.
Finding Deals and Understanding Your Warranty
Getting a good deal is always a plus, but the real value comes from a product’s long-term reliability. Some retailers may offer price matching or seasonal promotions, but a strong warranty is far more important. A warranty is a manufacturer's promise to stand by their product's quality and engineering. At Crawford Performance, we back our performance parts with a unique 12-month/12,000-mile engine warranty, giving you confidence that your investment is protected both on the track and on the trail. Always check the warranty details before you buy, as it speaks volumes about the manufacturer's trust in their own product.
AOS Maintenance: What to Do After Installation
One of the best things about a modern, well-designed AOS is that it’s largely a "set it and forget it" modification. Unlike old-school catch cans that require you to manually drain collected oil, a high-quality AOS automatically separates the oil and returns it to the engine crankcase. This closed-loop design means there’s no regular emptying or cleaning required on your part. The system works continuously in the background to protect your engine, keeping your intake clean and your intercooler efficient without adding another maintenance task to your list. This simplicity makes it one of the most effective and hassle-free upgrades you can make for your Subaru.
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Frequently Asked Questions
Is an AOS only for heavily modified Subarus? Not at all. While an Air Oil Separator is essential for tuned and high-performance builds, even a completely stock Subaru can benefit. The direct-injection engines in newer models are particularly prone to carbon buildup on the intake valves, which an AOS helps prevent. Think of it as a preventative health measure for your engine, ensuring its longevity and efficiency regardless of your power level.
What's the real difference between an Air Oil Separator and a catch can? The main difference is that an AOS is a complete, closed-loop system, while a catch can is a simpler, open-loop solution. A catch can does just what its name implies—it "catches" the oil and gunk in a container that you have to manually empty on a regular basis. An AOS, on the other hand, separates the oil and automatically drains it back into your engine's oil pan, making it a true set-it-and-forget-it upgrade with no maintenance required.
Will installing an aftermarket AOS void my car's warranty? This is a common concern, but installing an AOS should not automatically void your vehicle's warranty. Legally, a manufacturer has to prove that the aftermarket part you installed was the direct cause of a specific failure. Since an AOS is designed to protect your engine by improving on the factory's ventilation system, it's difficult to argue that it would cause damage. However, it's always a good idea to speak with your dealership's service department if you have concerns.
Do I need to get my car retuned after installing an AOS? Generally, no. An AOS is considered a reliability modification, not a performance part that alters your engine's air-fuel parameters. It simply ensures your engine is breathing clean, oil-free air, which allows your existing tune to perform as it was intended. It corrects a weakness in the factory system rather than changing how the engine is meant to run, so a retune is not necessary.
Are there any special considerations for using an AOS in cold climates? Yes, this is an important factor. In freezing temperatures, condensation can mix with the oil vapor inside the AOS and create a thick sludge that can block the drain-back line. To prevent this, some AOS systems are designed to be heated by your engine's coolant. This keeps the unit warm enough to ensure the separated oil stays fluid and drains properly back into the engine, providing reliable protection year-round.