Although I don't run an after market air filter, K&N is getting a lot of attention here. c400ex1234, says, on the box of the K&N filter reads (use the dirt build up to your advantage). I work in an auto plant and we use this principle to clean a liquid, used to cool machine tools, like drills and cutters. Build up a filter cake (dirt) on the outside of the filter element so that the larger particals get trapped against the filter and prevent the smaller particals from migrating through the filter. What they're not telling you is this is not "FILTERING". It's called "STRAINING". That's because until the filter builds a cake, it is bypassing particals right through the filter element no matter how much area the filter has. But it's probably not enough to be very harmful to your quad. Three to five microns is nothing. You can't see a partical with the naked eye until the partical is size 40 micron. But, when you get a lot of those small microns in the air, like riding behide somebody, this could be a killer to your engine. Think it's just up to the individual. I'll stick to my stock filter, myself.
Chuck, Suzuki KQ 300, GPS & lunch...