I know the K&N fans will probably jump on me, but that's ok. I know what I'm talking about and had more than enough proof...
I live in an area with paved roads and gravel. Which road I take depends on the weather and what I'm driving. My first car and favorite so far is my 1986 Pontiac Fiero GT fastback. It only catches fires every other week, so it's fun to drive. Before I got it painted, I used to take the gravel road with it sometimes so I could 4 wheel drift around the turns. Then one day I decided to get a K&N.
When I pulled my intake, it had just a little gunk built up in it, blackish stuff from the PCV. That was before I changed my valve stem seals. Put the K&N on and decided to check it in a few weeks. You know what I found? Junk caked EVERYWHERE, and a stalactite of dirt nearest the engine. On the POST-filter side.
If you've ever driven much on gravel, I'm sure you know what stuff I mean. It's this thick, caked, clay like mud formed from ultra-fine limestone particles trapped in oil. You'll find it anywhere you have an oil leak, spill, or forgot to wash after lubing things. It was all over the filter because it's an oil bath filter, but it bypassed the filter! And it wasn't just clinging to the post-filtered side. It was caked 1/8 to 1/4 inch thick on my intake tube as well from the PCV puking into my intake, proving that the dust was making it PAST all of that, even with a massively dirty filter which is supposed to filter better. Where the PCV goop just sucked down into the intake before, the dirt was causing it to build up.
But where did that dirt come from? Straight through the brand new K&N. Enough to cake up 1/8 to 1/4" thick all over! Defective filter, obviously, so I brought it back to the store. They said the warranty didn't cover consequential damages, blah blah, I said I just wanted a new filter. I led the guy to my car and showed him. His response? "Holy SH**!!"
Filter was exchanged, cleaned my intake, installed the new filter. Checked a week later, and the same thing was happening. What the heck? Exchanged again at the store, making sure they ordered a different batch from K&N after a lengthy phone call to the company. Same thing happened. So I used the kit to clean and re-charge my THIRD K&N, and it happened yet again.
I wasn't abusing the filter or cleaning it wrong. I didn't damage anything. It wasn't just one defective part, or two, or three brand new defective filters from different batches. The difference with this car was the slight amount of oil that allowed me to finally SEE the MASSIVE amounts of sh** the K&N was allowing directly through the filter all along, like it wasn't even there. I wish I had pictures, but I don't. I didn't own a nice camera at the time. I don't think I have anything to prove anyway though because I have not a thing to gain. Just sharing some facts.
Normally you don't get to see that stuff, so you assume it's all getting trapped. Bull. If you're okay with running an engine like that, cool. A lot of people run them on all kinds of cars and bikes without noticing any problems. Maybe they're accelerating wear. Maybe not. But the dust sized particles of ROCK coming through my filter showed me one thing: K&N don't filter worth a damn. Sorry, they just don't, plain and simple.
They do flow well though...
Keep the shiny side up. Cheers.