I have a K&N in my Fox and an MSDS clean air act system which uses K&N
type cone filter.  I am happy with both, the one in the fox uses the
regular air box, I didn't notice any changes in performance.  I cleaned
it at apx. 15k miles, it could have gone longer.  I drive on gravel/dirt
roads quite often.

My Vanagon on the other hand has the filter exposed and needed cleaning
at 5k.  The filer was only dirty on one side, which faces the engine.
This setup changes the sound of the engine a lot, I cannot tell with the
performance because I put it on a fresh rebuild.

dave