I used to run K&N's on my race quads. Granted this is like a worst case scenario. It was in an open air box running around a dusty track. I was METICULOUS with my filter maintenance. I would clean and re-oil after EVERY ride. Well after a season of MX, I noticed my quad smoking on start up and when I stabbed the throttle. I started paying way more attention to what was happening. I could take the K&N off after a race and run a clean white rag through my intake and pick up little pieces of dirt/sand. I started trying to clean more thoroughly and also tried about 6 or 7 different filter oils. NOTHING got the dirt to stop. I then went and bought some UNI foam filters. Low and behold... no more dirt in my intake. They were also less expensive, and easier to clean/re-oil. Now just imagine how this works on a boosted engine, as opposed to my naturally aspirated quad motors... not to mention the oiled filters seem like a bad idea in general for turbo diesels.
I will never use a K&N on anything I own again. For my truck now I use an AFE proguard 7 dry filter. Love it.