I run a K&N on my truck. It sees mostly street use, and only light off-road (I use it to tow the quad trailer out to desert campsites, sometimes as far as 10 miles). An outerwears is better than nothing, but it is just ONE DIMENSIONAL. If something doesn't get caught right off, it is going right on through. Same for the thin K&N.

For some extra peace of mind, I run a wrap called an "AIR FORCE FILTER" (K&N cat # 25-3924). It is a very coarse foam, that you oil and wrap around the K&N. It's a little messy to handle, but so is a blown engine. The great thing about foam is that it is MULTI DIMENSIONAL. If something doesn't get caught in the first layer, it is going to get caught further down the line. With thin stuff that only has one layer, you only have one chance to catch it. Plus, with foam the air doesn't take a straight line. Now, the K&N's laminar flow may be better for performance, but the airpath through the gause is straight. With foam, the air has to turn many corners, so it has way more chances to have the dirt caught.