Is an AOS or catch can worth installing on a stock 2022+ WRX?

A catch can is worth considering on a stock VB. A full air oil separator usually is not. The distinction matters and most threads blur it. Here is the underlying problem. The FA24 is direct injected, which means fuel is sprayed straight into the cylinder rather than across the back of the intake valves. On a port injected engine, fuel constantly washes the valves clean. On a direct injected engine, nothing does. Meanwhile the PCV system routes crankcase blow-by - which carries oil vapour - back into the intake tract. That oil vapour lands on intake valves that never get washed, bakes on, and becomes carbon deposits over tens of thousands of miles. This is not a WRX flaw, it is a characteristic of nearly every modern direct injected engine. Subaru's PCV control on the FA24 is meaningfully better than the older EJ engines, so the VB is not a worst case. But without port injection, the mechanism is still there. A catch can intercepts the blow-by before it reaches the intake and traps the oil in a small reservoir you empty periodically. It is simple, relatively cheap at roughly $210 to $280 for a quality baffled kit, and it directly addresses the problem. An AOS does something similar but drains the separated oil back into the sump rather than trapping it, and is typically heated to stop condensation freezing. That heating matters in genuinely cold climates and on track cars generating serious blow-by. On a street-driven stock VB it is solving a problem you do not have, at a higher price and with a more involved install. So the honest recommendation depends entirely on how long you intend to own the car. If this is a three-year lease or you will sell before 60,000 miles, skip both - the deposits will not have meaningfully accumulated on your watch. If you plan to keep the car past 100,000 miles, a catch can is cheap insurance against a walnut blasting service later, and it is one of the few genuinely preventative modifications available for this engine. Two practical points. First, a catch can is maintenance, not a fit-and-forget part. It needs emptying, more often in cold weather when condensation collects, and an ignored catch can that overfills is worse than no catch can. Second, buy a baffled design from a known manufacturer. An unbaffled can is a jar with hoses attached and separates very little. Neither part requires a tune, neither will set a check engine light when properly installed, and neither adds power. This is a longevity decision, not a performance one.