Let’s start with the basics. Your car uses a positive crankcase ventilation (PCV) system to manage pressure inside the crankcase and keep engine components from wearing out. The gasses that escape through the PCV system get routed back into the intake, but they don't come alone. They generally bring some oil mist along for the ride, and if you don’t control it, that extra oil can lead to serious engine issues.

We know what you’re thinking; venting the PCV system to the atmosphere sounds like an easy workaround. The problem is, not only is it illegal nationwide, but it also guarantees a smog-check failure and sends a fine oil mist into your engine bay and the environment. With that off the table, you’re left with two real solutions: an Air Oil Separator or a catch can—and they are not the same thing.

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The IAG AOS design incorporates a 1-liter cast aluminum swirl pot canister that significantly improves oil separation from the PCV system before returning PCV gasses to the intake.  

An Air Oil Separator is a substantial upgrade from a catch can, for a couple of reasons. While both remove oil from your PCV system and keep it from making its way back into the cylinders (we’ll get into why that’s bad soon enough), a catch can simply stores the oil, leaving you to either drain it periodically or pour it back into the engine. It’s a messy setup and can leave you running low on oil or recycling oil that’s already been contaminated.

An Air Oil Separator, or AOS for short, on the other hand, does the dirty work for you. After a simple installation, the Air Oil Separator goes to work on its own—circulating clean air back through the PCV system and returning the separated oil back to the pan. All you need to do is stay on top of your regular oil changes, and the AOS will handle the rest.

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Direct-injected Subaru engines—like the FA20F WRX and the newer FA24F—bring a few extra challenges to the table. Since DI doesn’t wash the back of the intake valves with fuel, any oil vapor that slips through the PCV system can stick and build up over time. That buildup chokes airflow, costs you power, and can bump up fuel consumption. Even port-injected Subaru engines benefit from cleaner PCV airflow, but the DI setups are especially sensitive, making an AOS a smart move if you want your engine running strong for the long haul.

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Unlike ordinary catch-can systems, the IAG Air / Oil Separator functions without the need for routine draining or maintenance.

Nothing ruins power—or engines—faster than detonation. When oil gets recirculated through your PCV (positive crankcase ventilation) system, it runs through your engine, collecting even more oil along the way. That excess oil can wreak havoc when it starts to cause poorly timed detonation, and when you’re putting your engine under heavy load—spirited driving, long climbs, track days—the problem only gets worse. Left unchecked, the added stress can snowball into serious wear or even catastrophic failure.

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Over time, oil buildup in the intake tube, charge piping, and manifold becomes almost unavoidable—but excess blow-by turns it into a much bigger problem. That oil coats the entire intake tract, cutting airflow and potentially reducing fuel octane along the way. Sooner or later, performance takes a hit, whether it’s a drop in horsepower, poor fuel economy, or a general loss of efficiency.

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Compared to a standard catch can, an Air Oil Separator does a much better job of keeping oil consumption in check. Every internal-combustion engine burns a little oil, but with a quality AOS, much of what would’ve been lost through the PCV system is routed back to the oil pan where it belongs.

A catch can, on the other hand, just collects that oil for disposal. Over time, that means you’re throwing away oil your engine could still use. Even the most consistent maintenance schedule can’t make up for running an engine that’s always a little low.

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You need an Air Oil Separator. It’s not the kind of upgrade you buy for bragging rights—it’s the one you buy because it works. Cleaner airflow means better fuel economy, more consistent power, and an engine that stays healthy far longer. For any Subaru that sees hard driving or daily miles, an AOS is a smart move that pays off every time you hit the throttle.

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Once you’ve made the call to run an IAG AOS, installation is straightforward thanks to the way the system is built. The canister uses custom-fitted SAE quick-connect lines cut to the exact length needed to reach each connection point, and most Subaru applications include pre-cut and crimped hoses for an even smoother install. For custom builds, IAG offers a wide selection of universal fittings that make it easy to build custom hose routes or replace existing connections. When it’s all in place, you’ll have a clean, reliable setup built to handle whatever your Subaru throws down.

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