Stock air box with a good OEM or Denso filters worked best. TRD or (K&N) reusable filters doesn't really filter out everything and I noticed my throttle body getting dirty quicker than OEM filters before I replace them.
I ordered and installed a k&n air-filter in my 19 Jetta r-line 1.4 and it seems to have a little more pep.
The k&n air filter is significantly thinner.
I personally have the k&n filter on my 2010 Forester and recently got the reusable cabin filter and have not had any problems.
I have one in both my Forester and wife's Crosstrek. Performance increase? Meh. On my XT it didn't matter, but I did find that the Crosstrek's acceleration from a stop increased nicely. I've had one on my Jeep for over 200k miles with no problems.
I use K&N filters on 3 of my vehicles - my '74 G30 350 V8 Chevy Van tow vehicle, my '93 RX-7, and my F2000 racecar. On the racecar, it is a required spec part required for all F2000 cars using the Zetec engine. In 14 race seasons (~100 races) of use it has caused no issues. My engine builder says my engine always looks good when they get it for a refresh. On the RX-7, it was part of an induction upgrade (the OE intake was very restrictive) and provided a noticeable power increase. It has caused no issues in the ~60K miles it's been in use. On the Chevy van, the OE air cleaner was way too small and restrictive for towing purposes. The K&N provided a significant benefit in gas mileage and power. It has been in use for ~200K miles and has caused no issues. YMMV.
I found a used ABD racing filter and I could feel it with out a doubt. I bought a K&N filter and drilled holes in my box and I say it was a waste.
Not a whole lot of difference, that is until you swiss cheese the air box!
IMO they're not worth it. Modern intake systems flow fairly well, adding a K&N filter will provide almost no benefit. Hell even adding an aftermarket intake these days add nothing but sound. they can also get oil residue on the MAF sensor which can be expensive to replace.
I had a green filter on for about 5k miles and I noticed it let a ton of pollen into the intake. I switched back to the OEM paper filter and the pollen was cut down substantially.
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