Best WRX Air Oil Separators and Catch Cans: VB, VA, and STI Picks

The best WRX air oil separator or catch can depends on which WRX you own and how much maintenance you are willing to do. A 2022+ VB WRX with the FA24DIT does not use the same kit as a 2015-2021 VA WRX with the FA20DIT, and an EJ-powered STI or 2008-2014 WRX is its own fitment path.

Quick answer: for most daily driven WRXs, a heated air oil separator is the best long-term choice because it separates oil from crankcase vapor and drains it back to the engine without a can to empty. The IAG AOS is the strongest default pick for VB and VA owners. The COBB AOS is the best VA choice if 50-state legality and COBB ecosystem support matter most. If you prefer to keep captured oil and water out of the sump, a Radium or Mishimoto catch-can kit makes more sense, but you must check and drain it.

Choose Your WRX AOS Or Catch Can

Why WRXs Need Oil Separation

Every turbocharged WRX has a PCV system that routes crankcase vapor back into the intake. That vapor carries oil mist, fuel vapor, moisture, and combustion byproducts. On a direct-injected FA20DIT or FA24DIT, fuel does not wash over the back of the intake valves, so oil vapor can contribute to intake-valve deposits over time.

Oil vapor in the intake charge is not ideal on a knock-sensitive turbo Subaru. An AOS or catch can reduces the amount that reaches the intake tract.

An AOS or catch can is not a power mod in the normal sense. You should not buy one expecting a noticeable horsepower gain. The value is cleaner intake plumbing, more consistent operation, less oil residue in the intercooler and charge pipes, and lower long-term carbon buildup risk. It can reduce the deposits that make FA20DIT intake-valve cleaning relevant, but it cannot replace inspection or physical cleaning when buildup is already present.

How We Picked These Products

We did not physically test every kit. We checked current manufacturer pages, retailer listings, product reviews, Subaru forum discussion, Reddit discussion, installation notes, and fitment documentation. We weighted correct fitment, serviceability, cold-weather behavior, emissions status, and long-term owner confidence over peak marketing claims.

Review volume is uneven. VA WRX products have years of owner history. VB products are newer, so we weighted current fitment, manufacturer support, and service design more heavily for 2022+ WRX recommendations.

Factor	Weight	What mattered
Correct WRX fitment	25%	VB FA24 vs VA FA20 vs EJ STI/WRX
Oil separation design	20%	Heated AOS, dual cans, PCV and CCV coverage, baffling
Service needs	20%	Periodic inspection for an AOS versus inspection and draining for catch cans
Owner and retailer signal	15%	Review counts, forum feedback, install complaints
Emissions and inspection friendliness	10%	CARB/50-state notes where available
Value	10%	Price, included hardware, warranty, install complexity

Best 2022+ VB WRX AOS And Catch Cans

Pick	Type	Best for	Main drawback
IAG AOS for 2022-2024 WRX	Heated AOS	Daily drivers that want low maintenance	Higher install effort and cost
Radium Dual Catch Can Kit	Dual catch cans	Owners who want to inspect and discard what is collected	Must be checked and drained
Mishimoto PCV Catch Can	Single catch can	Lower-cost VB protection	Only PCV side, manual draining

The IAG AOS is the best default recommendation for a 2022+ WRX because it solves the biggest catch-can problem: maintenance. IAG lists the FA24 kit for 2022-2024 WRX applications, with a 1-liter cast aluminum swirl-pot canister, pre-cut hoses, a mounting bracket, and color options. The design routes separated oil back to the engine rather than requiring you to empty a can.

The heated design is the practical reason to spend more. Cold-weather WRX owners should be cautious with unheated catch cans because moisture can collect, thicken, or freeze if the can is neglected. A heated AOS helps reduce condensation risk by bringing the separator closer to engine temperature.

Buy if: you daily drive your VB WRX, plan to tune it, drive in cold weather, or prefer an oil-separation setup that does not require routinely emptying a can.

Skip if: you want to inspect the collected oil/water mix, you dislike returning blow-by residue to the crankcase, or you want the easiest possible install.

Review evidence: VB owner volume is still newer than VA, but IAG’s Subaru AOS reputation, direct FA24 fitment, review count on IAG’s own page, and long history on older platforms make it the safest current VB pick.

Radium is the VB pick for owners who prefer catch cans over an AOS. Radium lists separate PCV, CCV, and dual catch-can kits for the 2022+ WRX. The dual kit covers both sides, uses Radium’s 7-fluid-ounce Fluid Lock cans, and includes VB-specific brackets and hardware.

The benefit of a catch can is that it traps oil, moisture, and fuel residue instead of returning it to the engine oil. The tradeoff is that you become part of the maintenance system. Radium specifically tells owners to start checking collection level after 3 to 4 weeks of driving and adjust the drain interval from there.

Buy if: you want to see what the engine is collecting, you prefer not to return separated contaminants to the sump, and you are disciplined about maintenance.

Skip if: you do not want to inspect and drain the cans, live somewhere very cold, or know you will neglect their service.

Review evidence: Radium has strong cross-platform reputation and clear engineering documentation. VB-specific long-term owner volume is still developing, so maintenance discipline matters more here.

Mishimoto’s 2022+ WRX catch can is the simpler budget path. Mishimoto lists it as a direct-fit PCV-side kit with pre-formed hoses, factory-style quick disconnects, a 3-ounce can, a 50-micron bronze filter, and a lifetime warranty.

This is not as comprehensive as a heated AOS or a dual catch-can kit. It is a lower-cost, easier-to-service way to catch oil from the PCV side. For a mostly stock VB WRX, that may be enough. For an ethanol, high-boost, track, or hard-use car, the IAG or Radium dual setup is easier to justify.

Buy if: you want an affordable direct-fit VB catch can and you are comfortable draining it during routine maintenance.

Skip if: you want CCV coverage too, you want no-maintenance operation, or you regularly drive in freezing conditions without checking the can.

Review evidence: Mishimoto published FA24 development notes and lists clear specs, but buyer-review volume is thinner than older VA products. Treat it as a clean budget fitment, not the most proven long-term choice.

Best 2015-2021 VA WRX AOS And Catch Cans

Pick	Type	Best for	Main drawback
IAG AOS	Heated AOS	Tuned daily VA WRXs	Cost and install complexity
COBB AOS	Heated AOS	50-state/CARB-conscious VA owners	More expensive
Radium Dual Catch Can	Dual catch cans	Owners who want to drain contaminants	Maintenance required
Mishimoto Dual Catch Can	Dual catch cans	Value-minded VA owners	Needs regular draining

The IAG AOS is the strongest overall VA WRX recommendation because it is proven, heated, and designed specifically for the FA20DIT WRX. IAG lists the kit for 2015-2021 WRX and 2014-2018 Forester XT, with a 1-liter cast aluminum canister and application-specific hoses.

Owners generally like that there is no catch can to empty. The hoses, fittings, coolant routing, and separator should still be inspected for leaks, damage, and condensation during normal service. IAG is also a common Subaru-shop recommendation for tuned daily drivers.

The common complaint is install effort. AOS installs take more time than simple catch cans, involve more hose routing, and can be frustrating if you are not comfortable removing intake plumbing.

Buy if: you want one long-term oil-separation mod for a VA WRX, especially a tuned car.

Skip if: you want the easiest install, lowest price, or the ability to discard collected contaminants instead of returning separated oil to the crankcase.

The COBB AOS is the best VA pick if emissions documentation matters. COBB lists its 2015-2021 WRX AOS as 50-state legal with CARB EO references, and says it was developed in partnership with IAG Performance. It uses a compact cast aluminum body, 1-liter capacity, integrated ORB ports, coolant heating, and a coolant bleeder valve.

That makes it a smart choice for a California or inspection-conscious owner who wants a premium AOS and clear documentation from a major tuning brand. COBB’s product Q&A also says the AOS does not require tuning when installed properly.

Buy if: you want a VA AOS with COBB support, CARB/50-state documentation, and no separate tune requirement.

Skip if: you are outside strict emissions states and the lower-priced IAG version gives you what you need.

Review evidence: COBB has fewer public product reviews than IAG’s own listing, but the emissions documentation and brand support make it uniquely useful for the right buyer.

The Radium dual catch-can kit is the best VA catch-can route if you want to service the system yourself and keep collected fluid out of the oil pan. MAPerformance lists the kit for 2015-2021 WRX, with two 7-fluid-ounce cans and factory mounting points. Radium’s own design language emphasizes baffling, sealed fittings, dipsticks, and clean installation.

Owner feedback is usually about quality and appearance. The downside is compatibility and service. Some owners report needing routing changes around aftermarket intakes, and Radium fitment notes warn that some applications, especially certain CVT/Lineartronic or modified-intake setups, need extra care.

Buy if: you want a premium dual-can setup and do not mind checking/draining cans.

Skip if: you do not want to inspect and drain a catch can, have a heavily modified intake path, or have a CVT car without verifying fitment.

Mishimoto’s dual catch-can system remains a good value pick for VA WRX owners. Mishimoto lists a two-can setup for the PCV and CCV sides, direct-fit brackets, billet aluminum sealed cans, 50-micron bronze filters, serviceable parts, and a lifetime warranty.

The main reason to choose Mishimoto is value and serviceability. You get a complete dual-can layout without stepping into a full heated AOS. Mishimoto’s engineering notes also claim the FA20DIT produces meaningful blow-by on both dirty and clean sides, which supports using a dual-can system rather than only a single PCV can.

Buy if: you want a complete dual-catch-can system for less than most premium AOS setups.

Skip if: you want no maintenance, drive in freezing weather without checking the cans, or want the most premium hardware.

Review evidence: Mishimoto has long-running FA20 product history and clear specs. Owner complaints tend to center on maintenance and installation details rather than the basic concept.

Best STI And 2008-2014 WRX AOS Picks

For EJ-powered WRXs and STIs, the IAG AOS is the safest default pick. IAG lists fitment for 2008-2014 WRX and 2008-2021 STI applications, and this platform has far more long-term AOS history than the newer VB.

The EJ crowd tends to use AOS systems on modified cars because ringland, oiling, boost, and track-use concerns are already part of the conversation. An AOS can reduce oil contamination in the intake path and keep the PCV system better controlled.

Buy if: you have an EJ WRX/STI, especially a tuned or track-used car.

The Grimmspeed AOS is the simple EJ-era option from the original version of this guide. It mounts at the oil-fill location and returns separated oil through that point, which makes the layout cleaner and easier to understand than some larger coolant-routed AOS systems.

The simplicity is also the drawback. It is not the same type of heated, multi-port, high-capacity setup as the IAG or COBB-style systems. For a basic older WRX or STI, it can still make sense. For a hard-driven or high-power car, choose a more comprehensive AOS.

Buy if: you want a simpler AOS layout for an older WRX/STI and your build is mild.

Skip if: you drive in cold conditions, make high boost, track the car, or want the strongest oil-control setup.

AOS Vs Catch Can: Which Should You Buy?

An air oil separator and a catch can solve the same broad problem in different ways.

Choice	How it works	Best for	Main responsibility
Heated AOS	Separates oil vapor and returns oil to the engine	Daily drivers, tuned cars, cold climates	More complex install
Catch can	Captures oil/moisture in a serviceable can	Owners who want to inspect and discard fluid	Regular draining
Single PCV catch can	Catches the dirty side only	Budget street cars	Less complete coverage
Dual catch cans	Catches PCV and CCV sides	Higher-boost or more thorough setups	More cans and hoses to maintain

For most street WRXs, a heated AOS is easier to live with. For owners who are disciplined about maintenance and do not like returning separated residue to the engine oil, a catch can is defensible.

The bad choice is a catch can you forget to drain. An overfilled or frozen catch can can create bigger problems than the oil mist you were trying to prevent.

Common Buying Mistakes

Buying the wrong chassis fitment: VB, VA, STI, and older WRX applications are different. Do not assume “WRX” is enough.

Ignoring PCV vs CCV coverage: a single PCV can is not the same as a dual-can setup or full AOS.

Using a cheap empty universal can: many low-cost universal cans have poor baffling, poor hose routing, or questionable fittings.

Forgetting cold-weather service: moisture is normal in crankcase vapor. If you use a catch can in winter, inspect it frequently.

Thinking an AOS fixes engine damage: excessive oil collection can indicate worn rings, overfilled oil, PCV problems, or other issues.

Skipping install instructions: hose routing matters. A wrong PCV or boost-reference connection can create idle, boost, vacuum, or oiling issues.

FAQ

Does a WRX AOS need a tune?

Usually no, when installed correctly. COBB’s VA AOS Q&A specifically says it does not need additional tuning. That said, any incorrectly installed PCV modification can create drivability problems, so follow the exact instructions for your kit.

Is an AOS better than a catch can?

For most daily driven WRXs, yes, because it is lower maintenance and usually better in cold weather when heated by coolant. A catch can is better if you want to inspect and discard what it collects.

Do I need one on a stock WRX?

You do not need one the way you need oil changes, tires, or brakes. It is a preventative mod. It becomes easier to justify on direct-injected FA20/FA24 cars, tuned cars, cars driven hard, and cars you plan to keep long term.

How often do WRX catch cans need to be emptied?

There is no universal interval. Radium recommends checking after 3 to 4 weeks of driving, then setting your interval based on how much your car collects. Winter, short trips, blow-by, and driving style can change the interval a lot.

Is a catch can or AOS emissions legal?

It depends on the exact product and state. COBB lists its 2015-2021 WRX AOS as 50-state legal with CARB EO numbers. Do not assume another kit has the same status unless the manufacturer lists it for your exact application.

Should I keep old oil from a catch can out of my engine?

That is the main argument for catch cans. They let you remove oil, moisture, fuel vapor, and sludge instead of returning separated oil to the crankcase. The tradeoff is that you must drain the can.

Bottom Line

If you own a 2022+ VB WRX, start with the IAG AOS if you prefer a return-to-sump design without a can to empty, or the Radium dual catch-can kit if you want to inspect and discard what the engine collects. If you own a 2015-2021 VA WRX, the IAG AOS is the best overall pick, while the COBB AOS is the best 50-state/CARB-conscious choice. If you own an EJ-powered STI or older WRX, the IAG AOS remains the most proven path.

The key is fitment and maintenance. Buy the kit for your exact engine, install it exactly as documented, and be honest about whether you will actually drain catch cans on schedule.