Does Your Subaru Need an Air Oil Separator?
You've invested in performance parts to make your Subaru faster, but there's a catch. As you increase power, you also increase engine blow-by. This sends a stream of oil vapor directly into your intake, contaminating the air-fuel mixture. This contamination effectively lowers your fuel's octane, forcing your ECU to pull timing and robbing you of the very power you worked so hard to gain. A high-quality air oil separator is a critical modification that protects your investment. It ensures your engine breathes clean, oil-free air, allowing it to produce consistent, reliable power every time.
Key Takeaways
- An AOS is a critical defense for your engine's health: It intercepts oily vapor from the crankcase, preventing the carbon buildup and detonation that robs your Subaru of power and can cause long-term damage.
- Choose a true AOS over a simple catch can: An Air Oil Separator is an active, closed-loop system that separates oil and returns it to the engine, making it a set-it-and-forget-it solution. A catch can simply collects oil and requires you to manually drain it.
- Maintain consistent power and reliability: By keeping your air-fuel mixture clean, an AOS ensures your fuel's octane rating isn't compromised. This allows your engine to produce reliable power, making it a foundational upgrade for any modified or hard-driven Subaru.
What Is an Air Oil Separator and How Does It Work?
If you’ve spent any time in the Subaru community, you’ve probably heard people talk about installing an Air Oil Separator, or AOS. So, what exactly is it? Think of it as a guardian for your engine. Your engine naturally produces something called "blowby," which is a mix of gases and oil vapor that slips past the piston rings and into the crankcase. An Air Oil Separator is a device designed to intercept these gases before they get recirculated back into your engine's intake.
Its job is to separate the oil vapor from the air. By doing this, it prevents that oily mist from coating your intake manifold, valves, and intercooler. This keeps your engine running cleaner and more efficiently, protecting it from carbon buildup and a harmful condition called premature detonation, or "knock." For anyone pushing their Subaru, whether on the track or a trail, an AOS is one of the most effective performance parts you can install for long-term engine health and consistent power. It’s a simple concept that delivers serious protection.
How an AOS Cleans Your Engine's Air
The magic of an AOS happens inside its canister. As the blowby gases from the crankcase enter the separator, the device uses a series of baffles or filters to create a swirling, vortex-like effect. This motion forces the heavier oil and fuel droplets to fling outward, where they stick to the walls of the canister. Gravity then takes over, and the collected oil drains down and is safely returned to the engine's oil pan. Meanwhile, the now-cleaner air continues its journey back into the intake system to be burned during combustion. It’s a continuous, self-contained process that ensures only air—not an oily cocktail—is making its way into your engine.
AOS vs. Stock PCV: What's the Real Difference?
Your Subaru already has a system to handle crankcase pressure: the Positive Crankcase Ventilation (PCV) system. Its job is to vent blowby gases out of the crankcase and route them back into the intake. While this works fine for a stock daily driver, it has a major drawback for performance-tuned engines. The factory PCV system sends all that oily vapor directly into your engine's intake. This oil contaminates the air-fuel mixture, effectively lowering your fuel's octane rating. This can lead to carbon buildup on your valves and pistons and increases the risk of engine knock, which can cause catastrophic damage over time. An AOS works with your PCV system, adding a critical filtration step that the factory setup lacks.
Is an AOS the Same as a Catch Can?
This is a common point of confusion, but an Air Oil Separator and a catch can are not the same. While they both aim to remove oil from crankcase gases, they function differently. A catch can is a simpler device that acts as a collection tank—it traps oil vapor and holds it in a canister that you have to manually empty on a regular basis. An AOS is a more advanced, active system. It doesn't just catch the oil; it separates it and automatically drains it back into the engine. This makes an AOS a true set-it-and-forget-it solution, ensuring your oil level stays consistent and eliminating the need for routine draining. You can learn more about the key differences on our blog.
Recirculating vs. Vent-to-Atmosphere Systems
When you're looking at Air Oil Separators, you'll find two primary designs: recirculating and vent-to-atmosphere (VTA). The difference is simple: a recirculating AOS routes the cleaned air back into your engine's intake, while a VTA system releases it directly into the engine bay or atmosphere. For a finely tuned machine like a Subaru, this choice is critical. Your engine’s computer (ECU) relies on a Mass Airflow (MAF) sensor to precisely measure the amount of air entering the system. A VTA setup can throw off these calculations by venting air that the MAF sensor has already accounted for, leading to an imbalanced air-fuel ratio, rough idling, and even a loss of power.
This is why a recirculating system is the gold standard for reliability and performance. By returning the cleaned air to the intake, it maintains the closed-loop system your ECU expects, ensuring smooth and consistent operation. There are no confusing signals sent to the computer, and you avoid the unpleasant smell of oil vapor in your engine bay. At Crawford Performance, we design all of our AOS kits as recirculating systems because they offer a no-compromise solution for protecting your engine. This approach is fundamental to creating reliable performance parts that work in harmony with your vehicle's sophisticated electronics.
Does Your Subaru Really Need an Air Oil Separator
Your Subaru’s engine is a finely tuned machine, but even the best engines have a natural byproduct called "blow-by." During combustion, a small amount of gas and oil vapor slips past the piston rings and into the crankcase. The factory PCV (Positive Crankcase Ventilation) system is designed to handle this by redirecting these gases back into your intake to be burned off. However, especially on a modified or hard-driven Subaru, this system can be overwhelmed.
When oil vapor enters your intake, it coats everything from the manifold to the valves with a layer of carbon and sludge. This buildup reduces airflow, lowers your fuel's octane rating, and can lead to detonation (engine knock), which robs you of power and can cause serious engine damage over time. An Air Oil Separator (AOS) is a critical upgrade that intercepts these vapors, separates the oil, and ensures only clean air is recirculated, protecting your engine and maintaining its performance.
How to Tell if Your Subaru Needs an AOS
Every engine produces blow-by, so this isn't about waiting for a problem to appear. Even a brand-new Subaru off the lot can show signs of oil buildup in its intake lines. The real question is how much your engine is producing and whether the stock PCV system can keep up. If you’ve modified your engine for more power, you’re definitely creating more blow-by. Telltale signs include increased oil consumption between changes or a fine oily film inside your intercooler or intake piping. For many enthusiasts, installing an AOS is a preventative measure to keep their engine clean from day one and maintain the performance they expect from their Subaru Crosstrek upgrades or other modifications.
Why Turbocharged and Direct-Injection Engines Benefit Most
While any engine can benefit from cleaner air, turbocharged and direct-injection (DI) engines, like those in modern Subaru WRX and Forester XT models, are particularly susceptible to the negative effects of blow-by. Turbochargers force more air into the engine, which creates higher cylinder pressures and, consequently, more blow-by. For direct-injection engines, the problem is compounded. In a traditional port-injection engine, gasoline is sprayed over the intake valves, which has a natural cleaning effect. DI engines inject fuel directly into the cylinder, so that oily vapor from the PCV system bakes directly onto the hot intake valves, leading to significant carbon buildup over time. An AOS helps your engine perform better by keeping this oil out, ensuring your fuel's octane rating isn't compromised and your valves stay clean.
Signs of a Failing Air Oil Separator
It's important to clarify that your stock Subaru doesn't come with a true Air Oil Separator; it relies on the PCV system. The following signs aren't necessarily indicating a "failing" part but rather that the factory PCV system is overwhelmed and failing to protect your engine. This is especially common in modified or hard-driven vehicles. If you notice these symptoms, it’s a clear signal that your engine is ingesting too much oil and would greatly benefit from an AOS installation. Think of it as your engine asking for help to breathe cleaner and run stronger, protecting your investment in performance parts.
Increased Oil Consumption
Are you finding yourself adding a quart of oil between scheduled changes? While some oil consumption can be normal, a noticeable increase often points to a problem. If there are no visible leaks on your garage floor, that oil has to be going somewhere. In many cases, it's being sucked through the PCV system as vapor, sent through your intake, and burned up during combustion. This is a classic sign that the stock system can't keep up with the amount of crankcase pressure your engine is producing, a common issue for tuned Subaru Impreza and WRX models.
Visible Exhaust Smoke
A puff of blue or grayish smoke from your exhaust, especially during a cold start or under heavy acceleration, is a telltale sign that your engine is burning oil. This is a direct result of the oil vapor that the PCV system allows into the intake manifold. When that oil enters the combustion chamber, it burns along with the air and fuel, creating visible smoke. While it might seem like a minor issue, it’s a clear indicator that your air-fuel mixture is being contaminated, which can lead to more significant performance and reliability issues down the line if left unaddressed.
Reduced Engine Performance
This is perhaps the most critical symptom for any enthusiast. When oil enters the combustion chamber, it effectively lowers your fuel's octane rating and increases the likelihood of engine knock, or detonation. Your Subaru's ECU is designed to detect this and will protect the engine by pulling ignition timing. This protective measure directly results in a loss of horsepower and torque. You might feel this as sluggishness, a lack of responsiveness, or just a general sense that the car isn't pulling as hard as it should. Essentially, oil contamination is robbing you of the very performance you're trying to achieve with your Subaru STI upgrades.
Why Don't Manufacturers Install an AOS from the Factory?
This is a fair question. If an AOS is so beneficial, why isn't one standard equipment? The answer comes down to a balance of cost, complexity, and priorities. For an original equipment manufacturer (OEM) like Subaru, the primary goals are meeting strict emissions regulations and fuel economy standards for the average driver, all while keeping production costs low. The factory PCV system is sufficient to meet these requirements for a stock vehicle under typical daily driving conditions. Manufacturers aren't engineering their cars for the enthusiast who plans to hit the track or add significant power. An AOS adds cost and another maintenance point, which doesn't align with the goals of mass production. That's where the aftermarket steps in, providing solutions like the Crawford Performance AOS to serve owners who demand more from their vehicles.
How an AOS Impacts WRX & STI Performance
An Air Oil Separator directly contributes to consistent engine performance by keeping your intake system clean. When oil vapor is allowed to enter the combustion chamber, it lowers the effective octane of your fuel, making your engine more susceptible to knock. Your ECU will then pull timing to protect the engine, which results in a noticeable loss of power. By installing an AOS, you ensure that only a clean air and fuel mixture enters the cylinders. This helps maintain your fuel’s octane rating, prevents carbon buildup on valves and pistons, and allows for more reliable and consistent power delivery. It’s an essential supporting modification for any performance-focused Subaru STI upgrades, ensuring your engine runs as efficiently and powerfully as possible.
How an AOS Protects Your Engine Investment
Think of an Air Oil Separator as an investment in your engine's longevity. The problems caused by oil in the intake—like carbon buildup, reduced fuel economy, and power loss—only get worse over time. In the worst-case scenario, excessive oil contamination can lead to detonation severe enough to cause catastrophic engine failure. An AOS prevents these issues at the source. It keeps your engine’s internal components clean, reduces wear, and helps you avoid costly repairs down the road. Whether you’re building a daily driver, a track car, or an overlanding rig, installing one of these performance parts is one of the smartest moves you can make to protect your Subaru’s heart and keep it running strong for years.
How to Choose the Right Air Oil Separator
Picking the right air oil separator is about more than just finding one that fits. It’s about matching the technology to your specific Subaru and how you drive it. A daily driver has different needs than a track-prepped STI or an overlanding Crosstrek. When you’re ready to buy, you’ll want to consider four key things: vehicle-specific fitment, the internal design, key features that simplify maintenance, and the quality of the materials. Getting these details right ensures your AOS does its job effectively, protecting your engine and maintaining performance for years to come. Let’s break down what you need to look for to make a smart choice.
Matching an AOS to Your Subaru Model Year
First and foremost, your AOS must be compatible with your car. While universal kits exist, a Subaru boxer engine has unique characteristics. That’s why it’s crucial to choose a model specifically designed for your vehicle, whether you have a WRX, STI, Forester, or Crosstrek. An AOS engineered for your exact year and engine type will have the correct mounting points, hose lengths, and flow capacity. This eliminates guesswork during installation and guarantees it functions correctly with your stock PCV system, providing the best possible protection against oil consumption and carbon buildup from day one.
Dual-Chamber vs. Single-Chamber: What's the Difference?
Not all air oil separators are created equal on the inside. The internal design directly impacts how well it works. Older, simpler systems often use a single chamber to try and separate oil from air, which can be moderately effective. However, modern designs like the Crawford Performance V3 AOS use a dual-chamber system. This advanced approach is far more efficient at capturing oil vapor and other contaminants before they reach your intake. For a performance-oriented engine that sees high RPMs and temperatures, a dual-chamber design provides a superior level of defense, keeping your intake cleaner and your engine running stronger.
Essential Features of a Quality AOS Kit
Beyond the core design, certain features can make an AOS more effective and easier to live with. Look for a system with an efficient internal baffling design, which helps knock oil vapor out of the air. Another key feature is the drain system. A good AOS should drain the collected oil back into the crankcase, creating a maintenance-free system you can set and forget. Cheaper catch cans require you to manually drain them, which is an easy chore to forget. Considering how an AOS integrates with other performance parts is also important for ensuring a clean and functional engine bay.
Heated vs. Non-Heated Designs
One of the biggest issues with poorly designed separators is sludge. When hot oil vapor mixes with condensation inside the canister—a common problem during short trips or in colder climates—it can form a thick, milky sludge. This gunk can clog the lines and render the entire system useless, negatively impacting engine performance. A heated AOS solves this problem by circulating engine coolant through the body of the separator. This keeps the unit warm enough to evaporate any water vapor, preventing condensation from forming in the first place. This ensures the oil drains back cleanly and the system functions correctly year-round, making it a critical feature for any daily-driven or all-weather Subaru.
Built-in Serviceability Features
A true Air Oil Separator is designed to be a "set-it-and-forget-it" solution. Unlike a basic catch can that simply collects oil and requires you to manually drain it, a quality AOS is an active system. It features an integrated drain that automatically returns the separated oil to the engine's crankcase. This self-draining function means you never have to worry about emptying a canister or the system overflowing. It creates a closed-loop, maintenance-free setup that ensures your engine is consistently protected without adding another task to your maintenance checklist. This is a key distinction that makes an AOS a superior long-term investment for your performance parts ecosystem.
Why You Shouldn't Skimp on AOS Materials
An AOS lives in a harsh environment—your engine bay—where it’s exposed to extreme heat and oil. That’s why build quality is non-negotiable. Look for units constructed from high-quality materials like billet, aircraft-grade aluminum. These materials are strong enough to withstand intense heat cycles and vibrations without cracking or failing. A well-built AOS is a long-term investment that reliably protects your engine whether you’re on a race track, a trail, or just commuting to work. Cheaper, poorly made alternatives can become a point of failure, so it pays to prioritize durability.
Understanding the Costs: AOS vs. Catch Cans
When you're looking at engine protection, the price tag is often a deciding factor. A catch can is typically the cheaper option upfront, but that lower cost comes with a trade-off: manual maintenance. You have to remember to drain it regularly, and if you forget, it overflows and sends oil right back into your intake—defeating its entire purpose. An Air Oil Separator, while a larger initial investment, is a complete, automated solution. It actively separates oil and returns it to the engine, making it a true "set-it-and-forget-it" system. The higher price reflects the more complex engineering that provides continuous, reliable protection without requiring you to pop the hood. For anyone serious about their Subaru's health, the cost of an AOS is an investment in peace of mind and the longevity of all your other performance parts.
Installing and Maintaining Your Air Oil Separator
Once you’ve chosen the right air oil separator for your Subaru, the next step is getting it installed and setting up a simple maintenance plan. Proper installation is critical for the AOS to do its job effectively, and thankfully, routine upkeep is incredibly straightforward. Taking the time to get this right ensures your engine gets all the protection and performance benefits you’re looking for. Think of it as the final step in future-proofing your engine against oil blow-by and intake gunk.
Typical Installation Time and Tools
So, what does it take to get an AOS installed? For most Subaru models, you can expect the installation to take around three hours. It's a perfect weekend project that doesn't require a professional lift or a massive tool collection. The process is more about patience than complexity, as you'll be routing hoses and working in some tight spaces. You'll need a basic set of tools, including a socket set (10mm and 12mm sockets will be your best friends), some needle-nose pliers for hose clamps, and zip ties to keep everything tidy. High-quality kits, like the ones we've developed for various Subaru WRX upgrades, come with detailed instructions to walk you through every step, ensuring you get a clean and correct installation.
Avoid These Common Installation Mistakes
Before you start turning wrenches, take a moment to prepare. Lay out all your tools, read the installation instructions from start to finish, and make sure you have a clean, well-lit space to work. One of the most common installation mistakes is adding oil to the separator before all the hoses are connected. This can cause oil to be immediately sucked into the intake, which is exactly what you’re trying to prevent. Always follow the specific sequence outlined in your kit’s instructions to avoid simple errors that can cause big headaches later.
Getting Hose Routing and Mounting Right
How you route the hoses is just as important as the AOS itself. Improper routing can lead to kinks that restrict airflow or clogs from built-up residue, making the system ineffective. When installing your kit, make sure the hoses have smooth, gentle bends and are kept away from hot engine components like the turbo or exhaust manifold. Securely mounting the AOS unit is also key; it should be in a stable location that’s still accessible for any future maintenance. A quality kit designed for Subaru STI Upgrades will include pre-cut hoses and model-specific mounting brackets to make this process much easier.
Your Simple AOS Maintenance Routine
The good news is that maintaining your AOS is simple. For catch-can-style systems, you’ll generally need to empty the collection chamber every 3,000 to 5,000 miles, which lines up perfectly with your regular oil change schedule. However, many modern systems, like our Crawford Performance V3 AOS, are designed to be virtually maintenance-free. These systems safely drain the collected oil and condensation back into the engine, so you can set it and forget it. This design eliminates the need for regular draining, giving you one less thing to worry about while protecting your engine.
Long-Term Service Intervals
A true Air Oil Separator is designed to be a long-term, low-maintenance solution for your engine. Unlike parts that require frequent checks and adjustments, a quality AOS is a critical defense for your engine's health that you can install and trust to do its job. It works by intercepting oily vapor from the crankcase, preventing the carbon buildup and detonation that robs your Subaru of power and can cause long-term damage. Because it automatically drains the collected oil back into the engine, there are no reservoirs to empty. Your only task is to occasionally inspect the hoses for cracks or wear during your routine oil changes. This "set-it-and-forget-it" design ensures your engine is continuously protected without adding another item to your maintenance checklist, making it one of the most practical performance parts you can install.
Catch Can Draining Schedule
If you opt for a catch can instead of a true AOS, you’ll need to add a new task to your regular maintenance routine. For catch-can-style systems, you’ll generally need to empty the collection chamber every 3,000 to 5,000 miles, which lines up with your regular oil change schedule. While this might sound manageable, it’s an easy step to forget. If the can fills up, it stops working, allowing oil vapor to flow right back into your intake—defeating its entire purpose. In colder climates, the collected water and oil mixture can also freeze and cause pressure issues. This manual process is the key difference that makes a self-draining AOS a more reliable and convenient solution for protecting your Subaru WRX upgrades and other performance investments.
Solving Common AOS Problems
Even with a perfect installation, you might have questions. If you notice a milky, yellowish substance in your AOS, don’t panic. This is usually just a mix of oil and water condensation, which is more common in colder climates or if you take frequent short trips where the engine doesn’t fully warm up. Another potential issue is a vacuum leak, which can cause a rough idle. If this happens, your first step should be to double-check that every hose clamp and connection is secure. If you’ve installed new performance parts and want to ensure everything is running in perfect harmony, professional tuning can dial in your setup.
Which Air Oil Separator is Best for Your Subaru?
When it comes to protecting your Subaru’s engine, not all solutions are created equal. After years of engineering and track-testing, we’ve developed what we believe is the most effective system for preventing oil contamination and preserving performance. Let’s walk through our top recommendation and see how it stacks up against other options on the market.
A Closer Look: The Crawford Performance V3 AOS
We engineered our V3 Air Oil Separator to be the ultimate defense for your engine. Its primary job is to intercept oily vapor from the crankcase before it can enter your intake system, preventing the harmful carbon buildup and oil consumption that robs your engine of power. By reducing engine blow-by by 78% compared to the stock system, the V3 AOS keeps your intake components clean and your engine running at its peak. We construct each unit from durable, aircraft-grade aluminum, ensuring it can handle extreme temperatures and the toughest driving conditions you can throw at it.
Track-Tested and Proven by Professionals
The ultimate test for any performance component isn't on a dyno—it's on the track, where every part is pushed to its absolute limit. Our V3 AOS was born from this environment, developed and refined through our work with motorsport legends like Ken Block for the demanding Gymkhana series. This isn't just a part we sell; it's the same technology that has protected some of the hardest-driven Subarus on the planet. On the track, maintaining consistent power is everything. By keeping the air-fuel mixture clean and preventing detonation, our AOS ensures the ECU doesn't have to pull timing, delivering reliable performance lap after lap. For anyone serious about pushing their Subaru, it’s one of the most effective performance parts you can install for long-term engine health and dependable power.
How Does the Competition Stack Up?
You’ll often hear catch cans mentioned in the same breath as an AOS, but they function very differently. A catch can is a simple container that collects oil, requiring you to manually empty it regularly. An Air Oil Separator, on the other hand, is a true closed-loop system. It’s designed to separate the oil from the air and safely return the clean oil back to the engine while recirculating the cleaned air into your intake. This makes an AOS a more integrated, lower-maintenance solution that you can essentially set and forget, unlike a catch can that adds another task to your maintenance checklist.
A Head-to-Head Feature Comparison
The long-term benefits of an AOS are where it truly shines. In our testing, Subaru engines equipped with a V3 AOS retain 95% of their original power after 50,000 miles. Engines without one can lose up to 15% of their power in the same timeframe due to carbon buildup and reduced efficiency. Our system is also designed to integrate seamlessly with your car’s factory components, so it won’t interfere with emissions compliance. Whether you're looking for Subaru WRX upgrades or parts for your Forester, an AOS is a foundational mod for engine health.
Real-World Performance: How Much Oil is Captured?
It's a fair question, but the answer isn't a simple number. The amount of oil an AOS captures depends heavily on your engine's health, your modifications, and how you drive. A turbocharged WRX or STI that sees regular track days will produce significantly more blow-by—and therefore capture more oil—than a stock Crosstrek used for daily commuting. However, the real measure of success isn't how much oil is collected, but what is prevented. Even a small amount of oil vapor, when constantly recirculated, can coat your intake valves and intercooler, leading to carbon buildup that chokes performance. The key benefit is ensuring that *none* of that oil contaminates your air-fuel mixture, which is crucial for maintaining consistent power and protecting your engine from detonation. By installing one of these essential performance parts, you're stopping the problem at its source, regardless of the exact volume captured during any given drive.
Installation Difficulty and Warranty Coverage
We designed our V3 AOS kits for a straightforward installation. Each model-specific kit comes with everything you need, and the process typically takes just 2-3 hours with basic hand tools. We stand behind our engineering with a comprehensive 12-month/12,000-mile warranty on all our performance parts. Our extensive experience in motorsports, including building race cars with Subaru, goes into every product we design. When you install a Crawford AOS, you’re not just getting a part; you’re getting a piece of race-proven technology backed by decades of expertise.
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Frequently Asked Questions
Is an AOS really necessary if my Subaru is completely stock? Yes, it’s one of the best preventative measures you can take. Every Subaru boxer engine, stock or modified, produces oil blow-by. While a stock engine produces less than a tuned one, that oily vapor still coats your intake valves and lowers your fuel's octane over time. Installing an Air Oil Separator on a stock car is an investment in its long-term health, keeping the engine clean and running efficiently from the very beginning.
What's the main advantage of an AOS over a much cheaper catch can? The biggest difference comes down to maintenance and effectiveness. A catch can is simply a reservoir that collects oil vapor, which you must manually drain every few thousand miles. If you forget, it can overflow and cause the very problem you're trying to prevent. An AOS is a complete, active system that separates the oil and automatically drains it back into the engine. This makes it a true set-it-and-forget-it solution that provides constant protection without adding another chore to your list.
Will installing an Air Oil Separator void my Subaru's warranty? Installing any aftermarket part carries some nuance regarding warranties. Legally, a manufacturer cannot void your entire warranty just because you installed an aftermarket part. They must prove that the specific part you installed directly caused the failure you are trying to claim. Since an AOS is designed to protect your engine from harmful oil contamination, it's considered a reliability modification that promotes engine health.
Do I need to get my car professionally tuned after installing an AOS? Generally, a professional tune is not required after installing an Air Oil Separator. The AOS is designed to work with your factory PCV system to improve its function, not fundamentally change how your engine operates. It simply ensures that cleaner air is entering your intake. This helps your engine run more consistently and reliably on its existing tune, whether it's the factory map or a custom one.
How can I tell if my AOS is working properly after installation? A properly functioning AOS works quietly in the background, so the best sign is the absence of problems. The most direct way to check its effectiveness is to inspect your intake tract or intercooler piping during other maintenance. You should find them to be dry and free of the oily film that is common in engines without an AOS. This cleanliness is the clearest indicator that the system is successfully capturing oil vapor before it can contaminate your engine.