Once you start adding performance parts to your WRX or STI, you're building a system where every component counts. You can have the best tune and the biggest turbo, but if your engine is breathing in oily air, you're not just losing power—you're risking detonation. This is why a Subaru air oil separator is a foundational upgrade, not an afterthought. It ensures your engine gets a pure air-fuel mixture, allowing it to produce consistent, reliable power under load. This guide is for the owner who demands the most from their machine and understands that engine protection is the key to achieving true performance.
Key Takeaways
- Guard Your Engine's Health: An Air Oil Separator is a critical defense for your Boxer engine, preventing oil vapor from contaminating your intake. This upgrade helps stop carbon buildup and engine knock, preserving your engine's power and reliability.
- Choose a Maintenance-Free System: A true AOS is a closed-loop design that automatically returns oil to your engine, unlike a catch can that needs constant draining. This "set it and forget it" functionality provides continuous protection without adding another chore to your list.
- An AOS is for Every Subaru Owner: Whether your engine is stock or heavily modified, it benefits from an AOS. It's a preventative measure for a daily driver and an essential safeguard for a performance-tuned vehicle, protecting your investment either way.
What Does a Subaru Air Oil Separator Actually Do?
If you’re serious about getting the most out of your Subaru, you’ve probably heard about the Air Oil Separator, or AOS. Think of it as a dedicated guardian for your engine. Its main job is to intercept oily vapor from your engine's crankcase before it can get into your air intake system. This process is vital for maintaining engine health, especially in high-performance and modified Subarus. By cleaning the air that re-enters your engine, an AOS helps ensure consistent power delivery and protects critical components from harmful buildup. It’s one of the most effective performance parts you can install to support both engine longevity and reliability, whether you're on the track or a backcountry trail.
A Look Inside: How Your AOS Works
Your engine’s crankcase builds up pressure from gases that slip past the piston rings—a phenomenon known as "blow-by." The Positive Crankcase Ventilation (PCV) system is designed to vent these gases back into the intake manifold to be burned off. The problem is, these gases are full of oil mist. An Air Oil Separator is installed between the crankcase and the intake to solve this. As the oily vapor passes through the AOS, it goes through a series of baffles or a centrifuge that causes the heavier oil droplets to separate from the air. The oil is then collected and drained back into the oil pan, while the now-clean air continues to the intake.
Is an AOS Necessary for Your Boxer Engine?
The flat design of the Subaru Boxer engine makes it particularly susceptible to oil blow-by, especially under the high G-forces of aggressive cornering or acceleration. When that oil vapor gets into your intake, it coats the inside of the manifold, valves, and intercooler, and lowers the effective octane of your fuel. This can lead to detonation (engine knock), reduced power, and increased carbon buildup over time. An AOS is a critical defense against this, ensuring your engine is only burning a clean air-fuel mixture. It’s a preventative measure that protects your engine from the long-term damage caused by oil contamination, making it an essential upgrade for any modified Subaru.
Air Oil Separator vs. Catch Can: What's the Difference?
You’ll often hear catch cans mentioned in the same breath as an AOS, but they are not the same. A catch can is a simpler device that also filters oil from crankcase vapor, but it just collects the oil in a reservoir that you have to empty manually. If you forget, it can overflow back into the system. More importantly, a catch can removes that oil from the engine entirely, which can lead to running low on oil between changes. An AOS is a closed-loop system. It separates the oil and automatically returns it to the engine, so there’s no manual draining and no risk of running your oil level down. For this reason, the Subaru community generally agrees that an AOS is the superior solution for reliable, long-term protection.
Guideline for Choosing Between an AOS and a Catch Can
Deciding between an AOS and a catch can really comes down to your driving habits and how your car is set up. For a completely stock daily driver that sees mostly city and highway miles, a catch can is a decent, budget-friendly option. It will capture a good amount of blow-by from normal driving, but you have to commit to emptying it regularly, usually with every oil change. However, if you have a track car, a high-performance build, or even a lightly modified Subaru that you drive aggressively, an Air Oil Separator is essential. These scenarios produce far more blow-by, and an AOS is designed to handle that higher volume without needing your intervention. It’s an active system that uses coolant lines to prevent sludge buildup and automatically returns clean oil to the engine, making it a true "set it and forget it" solution for serious performance parts.
Risks of an Improperly Maintained Catch Can
While a catch can is better than nothing, it comes with risks if not maintained perfectly. The biggest issue is overflow. If the can gets too full, the collected oil, fuel, and water sludge can get sucked right back into your intake manifold. This defeats the entire purpose of the device and can cause smoking, poor performance, and even contribute to engine knock. Another significant drawback is that a catch can permanently removes oil from your engine's system. Over the course of an oil change interval, this can lead to your engine running low on oil, which is a major risk for any engine, especially a high-strung turbocharged Boxer. This is why a closed-loop AOS is the superior choice for protecting your investment and ensuring your engine tuning is built on a healthy foundation.
Why Installing an Air Oil Separator is a Smart MoveAir Oil Separator?
Installing an Air Oil Separator (AOS) is one of the most effective upgrades you can make for the long-term health and performance of your Subaru’s Boxer engine. Think of it as a dedicated filtration system for your engine's crankcase ventilation. It intercepts the oily vapor, known as blow-by, that escapes past the piston rings and separates the oil from the air. The clean air is then sent back into your intake to be burned, while the collected oil is safely returned to your oil pan.
This simple process has a massive impact. It prevents oil from coating your intake manifold, valves, and intercooler, which keeps your engine running efficiently. By ensuring only clean air enters the combustion chamber, an AOS helps maintain your engine's power output and prevents a host of potential problems down the road. Whether you’re pushing your Subaru WRX on the track or navigating a remote trail in your Crosstrek, an AOS provides critical protection that pays for itself over time.
Extend Your Engine's Lifespan
An Air Oil Separator acts as a bodyguard for your engine’s internal components. By filtering out harmful oil mist and other contaminants from the crankcase ventilation system, it prevents that gunk from being recirculated into your engine's intake. This is crucial because oil vapor can lower the effective octane of your fuel, leading to detonation that can damage pistons and bearings. An AOS ensures a cleaner, more stable combustion process, which reduces wear and tear on critical parts. This simple but effective modification is a key step in preserving your engine’s health for years to come.
Gain a Performance Edge on Track and Trail
When you demand a lot from your Subaru, your engine needs to breathe clean air. Oil contamination in the intake tract can slowly degrade performance by reducing thermal efficiency and contributing to engine knock, especially under high load. An AOS keeps the air/fuel mixture pure, allowing your engine to produce consistent and reliable power. This is why so many enthusiasts who track their cars or depend on them for overlanding solutions consider an AOS an essential upgrade. It helps your engine perform at its peak, whether you're hitting an apex or climbing a steep grade.
Say Goodbye to Carbon Buildup
Over time, oil vapor circulating through the intake system bakes onto surfaces, creating hard carbon deposits. This buildup is particularly problematic on the intake valves of direct-injection engines, where it can restrict airflow and lead to rough idling, misfires, and a loss of power. Installing an AOS significantly reduces this carbon buildup, keeping your engine cleaner internally. This means less time and money spent on intensive maintenance procedures like walnut blasting to clean the intake valves. It’s a proactive measure that helps you avoid costly repairs and maintain smooth engine operation.
Why a Closed-Loop System Matters
One of the biggest benefits of a true Air Oil Separator is its closed-loop, maintenance-free design. Unlike a catch can, which simply collects oil and must be emptied manually, an AOS is a fully integrated system. It separates the oil from the air and automatically drains the collected oil back into the engine's crankcase. This "set it and forget it" functionality means you don’t have to worry about checking levels or draining a container. It ensures your engine is always protected without adding another task to your regular maintenance checklist.
Maintain Proper Crankcase Vacuum
Your engine's crankcase needs to maintain a slight vacuum to help the piston rings seal effectively and prevent oil leaks. However, the pressure from blow-by gases works against this. A quality Air Oil Separator is designed to manage this pressure correctly. As it vents the crankcase, it maintains a stable vacuum, creating an ideal internal environment for your engine to operate. By separating oil from the air before it re-enters the intake, the AOS ensures the PCV system functions as intended without introducing contaminants. This is a critical function that supports overall engine health and is a cornerstone of reliable performance parts integration, helping your engine run smoother and last longer.
Choosing the Best Air Oil Separator for Your Subaru
Once you’ve decided an Air Oil Separator is the right move for your Subaru, the next step is picking the right one. With several options on the market, it’s easy to get overwhelmed. The key is to look past the marketing and focus on the engineering, build quality, and design principles that will actually protect your engine. Different brands take different approaches, and understanding these nuances will help you make an investment that pays off in performance and longevity, whether you’re daily driving your WRX or pushing your STI to its limits on the track.
Crawford Performance: The Original Standard
When it comes to Subaru boxer engines, experience matters. The Crawford Performance Air/Oil Separator is widely recognized as the original and the gold standard in engine protection. Developed through years of rigorous testing on the track and trail, our design is engineered specifically to handle the unique demands of the Subaru platform. We pioneered the technology to solve the oiling issues inherent in these engines, and we’ve been refining it ever since. While many cheaper copies have entered the market, they often lack the sophisticated internal baffling and proven design that make our AOS so effective. Choosing Crawford means opting for a part that was created by Subaru performance experts, not just replicated.
A Legacy Born from Ken Block's Gymkhana Builds
The Crawford Performance Air Oil Separator isn't just another aftermarket part; its history is rooted in one of the most iconic series in motorsport. This technology was born out of necessity during the original Gymkhana builds for the legendary Ken Block. Pushing those cars to their absolute limits revealed a critical need for bulletproof engine protection, and Crawford Performance stepped up to invent the original AOS. This race-proven design was engineered to withstand the most demanding conditions by intercepting oily vapor from the crankcase before it could contaminate the intake system. This ensures only clean air reaches the combustion chamber, maintaining consistent power and protecting vital engine components. That legacy of innovation is why the Crawford Performance AOS is still considered the gold standard for protecting your Subaru's Boxer engine.
IAG Performance: Built for Power
IAG Performance is another well-known name in the Subaru aftermarket scene, and they offer a line of Air Oil Separators for models like the WRX, STI, Forester XT, and Legacy GT. Their products are a popular choice and provide another option for owners looking to address oil blow-by. Like other units on the market, their AOS is designed to capture oil vapors from the crankcase ventilation system. When comparing their offerings, it’s helpful to look at the specific features, materials, and installation kits for your vehicle to see how they stack up against other systems.
Perrin Performance: Style and Function
Perrin Performance also provides Air Oil Separators for the Subaru community. Their systems are designed with the goal of improving engine efficiency and longevity by managing oil vapor. They focus on creating a solution that separates oil from the PCV system before it can be recirculated into your intake. As with any major performance part, it’s a good idea to examine their design, including the can construction, hose materials, and mounting hardware. Understanding how their system integrates with your specific Subaru model, from an Impreza to a Forester, is a key part of the evaluation process.
Comparing Key Features: What Really Matters?
When you’re comparing different AOS units, the details make all the difference. Don’t just look at the price tag; consider the critical features that impact performance and reliability. First, check the quality of the fittings and hoses. Low-quality components can lead to leaks or failures down the road. Second, consider the ease of installation. A well-designed kit will come with clear instructions and vehicle-specific mounting brackets. Finally, pay close attention to the routing of the coolant and drain lines. Improper routing can cause the AOS to function poorly in cold weather or lead to maintenance headaches. These factors separate a weekend project from a professional-grade upgrade.
Don't Overlook Build Quality and Design
The effectiveness of an Air Oil Separator comes down to its internal design and overall build quality. An AOS is more than just an empty can; its job is to efficiently separate oil vapor from the air before it enters your engine’s intake. This is accomplished through a series of internal baffles and chambers that cool the vapor and allow the oil to condense and drain back to the engine. A superior design ensures maximum separation without impeding airflow. High-quality materials, like machined billet aluminum, ensure the unit can withstand engine bay heat and pressure for years. This is why investing in well-engineered performance parts is crucial for preventing oil loss and carbon buildup.
Unique Design Features of Different Brands
Let's be clear: the magic of an AOS happens inside the can. While many units look similar on the outside, their internal engineering is what sets them apart. The Crawford Performance AOS, for example, was developed through decades of motorsport experience and is specifically engineered to counteract the oiling challenges inherent in the horizontal Boxer engine. It uses a sophisticated internal baffling system that creates a centrifugal effect, forcing the heavier oil droplets out of the air and allowing them to drain back to the pan. Other brands may use different approaches, such as mesh media or simpler chamber designs. The most important question to ask is how effectively a unit functions under dynamic conditions, from high-G cornering to rapid acceleration. The internal design is the most critical of all performance parts, as it directly impacts how well your engine is protected.
Your Guide to AOS Cost, Installation, and Upkeep
Adding an Air Oil Separator to your Subaru is a smart investment in its long-term health and performance. Before you commit, it’s helpful to understand the costs, what the installation involves, and what kind of upkeep is required. This upgrade is more straightforward than you might think and offers significant peace of mind, whether you’re hitting the track or the trail. Let's walk through what you can expect when you decide to protect your Boxer engine with an AOS.
How Much Does a Subaru Air Oil Separator Cost?
The price for an Air Oil Separator can vary, but you can generally expect to invest between $300 and $600. For example, our Crawford Performance Air Oil Separators are typically priced up to $450, with specific installation kits available. Other brands like IAG often price their units around $500. The cost reflects the quality of materials, design complexity, and features like heating elements. Think of it not as an expense, but as an investment in preventing much more expensive engine issues down the road, like carbon buildup and reduced efficiency.
Can You Install an AOS Yourself?
One of the best things about an AOS is that it’s a "bolt-on" part. This means it’s designed to fit directly into your Subaru’s engine bay without requiring custom fabrication. If you’re comfortable working on your car, you can likely handle the installation yourself over a weekend. Most kits come with detailed instructions, and the process is fairly direct. For those who prefer to leave it to the experts, having a professional mechanic handle the installation is always a great option. Our team offers expert engine tuning services and can ensure your AOS is installed perfectly.
What Kind of Maintenance Does an AOS Need?
Here’s where a high-quality AOS really shines. Unlike a catch can that needs to be regularly emptied, a well-designed AOS operates on a closed-loop system. This means the separated oil is safely returned to the oil pan, so you never have to drain or service it. It’s a true "set it and forget it" modification. This maintenance-free design ensures your engine is constantly protected from oil blow-by without adding another task to your vehicle’s upkeep schedule. You get all the benefits of cleaner intake and better performance without any of the hassle.
Preventative Replacement and Long-Term Considerations
While an AOS is a durable component, it's best viewed as a long-term preventative measure. An Air Oil Separator acts as a bodyguard for your engine’s internal components. By filtering out harmful oil mist and other contaminants from the crankcase ventilation system, it prevents that gunk from being recirculated into your engine's intake. This is crucial because oil vapor can lower the effective octane of your fuel, leading to detonation that can damage pistons and bearings. By ensuring a clean air-fuel mixture, an AOS protects your engine from gradual damage, making it one of the most valuable performance parts you can install for long-term reliability.
Remember to Replace Hoses
When you install or service your Air Oil Separator, it's a perfect time to replace the associated hoses. Over years of heat cycles in the engine bay, rubber and silicone hoses can become brittle, hard, and prone to cracking. These cracks can create vacuum leaks, which can throw off your air-fuel ratio, cause rough idling, and trigger check engine lights. A small leak can completely undermine the benefits of your AOS and create frustrating tuning issues. Using high-quality replacement hoses ensures a tight seal, preventing unmetered air from entering your system and keeping your engine running smoothly and efficiently.
Potential for Oil Dilution Over Time
While a true AOS is a "set it and forget it" system, it's important to understand how it works. The system returns separated oil to the crankcase, but a small amount of fuel and water vapor can sometimes come along for the ride. This can slightly dilute your engine oil over time, which means you might need to adjust your maintenance schedule accordingly. For many owners, especially those who drive their cars hard, this simply means changing the oil a bit more frequently than the factory-recommended interval. Staying on top of your oil changes is always a good practice for a modified engine and a core principle of any good engine tuning strategy, ensuring your AOS can do its job effectively without compromising oil quality.
Matching an AOS to Your Subaru Model
Not all Air Oil Separators are created equal, and it’s crucial to get one that’s specifically designed for your vehicle. Whether you drive a WRX, STI, Crosstrek, or Forester, there’s a kit made for your engine. We make it easy to find the right performance parts by organizing them by model. When shopping, always confirm the part is compatible with your Subaru’s model and year. This guarantees a perfect fit, proper hose routing, and optimal performance, so you can get back to enjoying the drive.
Does Your Subaru Need an Air Oil Separator?
Deciding whether to install an Air Oil Separator isn't just a question for track day regulars or owners with heavily modified engines. The reality is that every Subaru, from a stock daily driver to a fully built race car, can benefit from the protection an AOS provides. The unique design of the Boxer engine makes it prone to circulating oil vapor through the intake system, which can lead to a host of issues over time. The key is learning to spot the signs that your engine is asking for help.
Some indicators are obvious, like a puff of smoke from the exhaust, while others are more subtle, such as a gradual decline in performance or efficiency. Understanding these clues will help you determine if an AOS is the right move for your vehicle. Whether you drive a WRX, STI, Crosstrek, or Forester, paying attention to your engine’s health is the first step toward preserving its power and reliability for years to come. This section will walk you through the key warning signs and help you decide if an AOS is a worthwhile investment for your setup.
Is Your Subaru Burning Too Much Oil?
If you find yourself topping off your oil between changes, it’s a clear sign your engine is consuming it. While some oil use is normal for a Boxer engine, significant loss points to a problem. This is where many people mistakenly believe a simple catch can is enough protection. However, a catch can only traps contaminants; it doesn't return the captured oil to the engine, increasing the risk of running the engine low on oil if you aren't constantly monitoring it. An AOS solves this by safely returning clean, usable oil to the crankcase, ensuring your engine is properly lubricated while preventing harmful blow-by from re-entering your intake.
Smoke and Power Loss: Telltale Signs
Visible smoke from your exhaust is a tell-tale sign that oil is burning in the combustion chamber. But performance issues can be a more subtle clue. If you have a turbocharged Subaru like a WRX or Forester XT with performance modifications, an AOS is crucial. Harmful oil vapors and contaminants in your intake manifold can effectively lower your fuel's octane rating, which can lead to detonation problems, also known as engine knock. This can manifest as hesitation, rough idling, or a noticeable loss of power. By installing an AOS, you ensure only clean air enters the intake, protecting your engine and maintaining the performance you expect from your Subaru WRX upgrades.
Other Symptoms of a Failing AOS
A failing AOS can sometimes mimic the problems of an overflowing catch can. If the unit becomes clogged or stops separating oil effectively, that collected gunk can be sucked directly into your engine. This defeats the entire purpose of the system and can lead to a range of issues you can't ignore. You might notice increased smoke from the exhaust, especially when you get on the throttle, or a general sluggishness in performance. A rough idle or even fouled spark plugs can also point to an AOS that isn't doing its job. These symptoms are your engine's way of telling you that it's breathing in oil instead of clean air, which is why investing in high-quality performance parts from the start is so important.
The Worst-Case Scenario: Engine Hydrolock
In a severe failure, a faulty AOS can allow a large volume of oil to enter the intake manifold and flood the cylinders. This can lead to a catastrophic event known as engine hydrolock. Because liquids like oil can't be compressed, the piston slams into a wall of fluid on its compression stroke. The resulting force has to go somewhere, and it can cause devastating internal damage, including bent connecting rods, cracked pistons, or even a destroyed engine block. While this is a rare occurrence, it's the ultimate consequence of a poorly designed or failed system. It underscores why choosing a proven, reliable AOS isn't just about performance—it's about protecting the very heart of your Subaru from catastrophic failure.
Common Installation Questions, Answered
With so many brands and designs on the market, it can be difficult to pick the right Air Oil Separator. It’s easy to wonder if the one you choose is really working as well as it should. A poorly designed or improperly installed AOS can be ineffective or, in some cases, even create new issues like vacuum leaks. That’s why it’s essential to select a unit from a reputable manufacturer that is engineered specifically for your Subaru model. Proper installation is just as important. If you’re not confident in routing the hoses and ensuring a perfect seal, it’s always best to have the work done by a professional who specializes in engine tuning services.
Is It Worth It for Stock vs. Modified Engines?
One of the biggest misconceptions is that an AOS is only for heavily modified, high-horsepower race cars. The truth is, every Subaru engine, right from the factory, can benefit from an AOS. The factory PCV system allows oil vapor into your intake, which over time leads to carbon buildup on your intake valves and pistons. For a stock vehicle, an AOS is a preventative measure that protects the long-term health and efficiency of your engine. For a modified engine, it’s an essential component. When you’ve invested in performance parts, an AOS protects that investment by keeping the air-fuel mixture clean and preventing power-robbing detonation.
Limitations on Heavily Modified Engines
The more power you add to your Subaru, the more you need an Air Oil Separator. Upgrades like a larger turbo, a high-performance tune, or increased boost all create higher cylinder pressures, which in turn leads to more blow-by. The Boxer engine's flat design already makes it prone to oil sloshing and blow-by under hard driving, and modifications amplify this effect. When that increased volume of oily vapor gets into your intake, it lowers your fuel's effective octane rating, which is a direct path to engine knock or detonation. This is especially dangerous in a tuned engine running on the edge of performance. An AOS isn't just a supporting mod; it's a foundational piece of protection for any modified Subaru, ensuring your engine gets the clean air it needs to make power safely.
How to Tell if an Air Oil Separator is Bad
Even the best parts can fail, and an AOS is no exception. The most common sign of a failing unit is a sudden increase in oil consumption. If you notice you're adding oil more frequently than you used to, your AOS may not be separating oil effectively. Another clear indicator is visible blue or white smoke from the exhaust, especially during acceleration, which signals that oil is burning in the combustion chamber. You might also notice oil residue or leaks around the AOS unit itself or the connected hoses. In some cases, a failing AOS can cause a rough idle or even trigger a check engine light if it creates a significant vacuum leak. Paying attention to these symptoms can help you catch a problem before it leads to more serious engine issues.
Technical Diagnosis with a Manometer
If you suspect your AOS isn't working correctly, the most accurate way to test it is with a tool called a manometer. This device measures the vacuum pressure inside your engine's crankcase. A healthy PCV system, supported by a functioning AOS, should maintain a specific, slight vacuum at idle. To perform the test, a manometer is connected to the engine's oil dipstick tube. If the reading shows excessive vacuum, it could mean there's a clog in the system. If there's little to no vacuum or, even worse, positive pressure, it indicates the AOS is failing to properly vent the crankcase. This technical diagnosis is the definitive way to confirm the health of your oil separation system.
When High Oil Consumption Points to Other Engine Issues
It's important to remember that an Air Oil Separator is designed to manage oil vapor from blow-by, not fix internal engine problems. If you install a high-quality AOS and find that your Subaru is still consuming a significant amount of oil, it may be a sign of a more serious underlying issue. Conditions like worn piston rings, failing valve seals, or a damaged turbo seal can also lead to high oil consumption. In these cases, the AOS is doing its job, but the oil is being lost through other means. If your oil loss continues after installing an AOS, it’s a good idea to seek a professional diagnosis to identify the true source of the problem and protect your engine from further damage.
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Frequently Asked Questions
Is an AOS really necessary for a stock Subaru that I just daily drive? Yes, absolutely. Think of it as a preventative measure for your engine's long-term health. Every Boxer engine produces oil blow-by, even a brand new one. An Air Oil Separator prevents that oily vapor from building up as carbon on your valves and inside your intake over time. Installing one early helps maintain the engine's original performance and efficiency for years to come, making it one of the smartest reliability upgrades you can make, regardless of your power level.
Can you explain the difference between an Air Oil Separator and a catch can one more time? Of course. The simplest way to think about it is that a catch can is a dead-end trap, while an AOS is a complete recycling system. A catch can collects the oil in a container that you must manually empty. If you forget, it can overflow. An AOS, on the other hand, separates the oil and automatically drains it right back into your engine's oil pan where it belongs. This makes it a true "set it and forget it" solution that requires no maintenance.
Will installing an aftermarket AOS void my Subaru's factory warranty? This is a common concern, but generally, no. In the United States, your warranty is protected by the Magnuson-Moss Warranty Act, which states that a dealer must prove an aftermarket part directly caused a specific failure to deny a warranty claim. Since a high-quality, properly installed AOS is designed to protect your engine from known issues, it's highly unlikely to cause a problem that would put your warranty at risk.
I've heard an AOS can have issues in the winter. Is that true? This can be a problem for poorly designed units, but not for a well-engineered one. In freezing temperatures, the moisture and oil vapor in the system can thicken into a sludge that causes a blockage. Reputable systems solve this by using engine coolant lines to heat the AOS unit. This keeps the contents warm and fluid, ensuring it functions perfectly year-round, no matter how cold it gets outside.
Why do some AOS kits cost so much more than others? The price difference comes down to engineering, materials, and effectiveness. A less expensive unit might just be a simple can with basic filters, while a premium AOS is a highly engineered part with complex internal baffles designed for maximum oil separation. The cost also reflects higher-quality materials like machined aluminum, better hoses and fittings, and vehicle-specific brackets that ensure a perfect, leak-free installation. You're investing in a part that is proven to protect your engine reliably.