k&n air filters...

[Dunenutt]

I have them on everything I own.

Secret to a K&N is, dont over oil them ( when you buy one look at it and remember how red it is), and dont over clean them. Most of mine get cleaned once a year, and I spend alot of time off road! Even if it looks dirty, it still passes WAY more air than a paper filter.
I did a test a few years back on my Cummins. I used a Magnahelic gauge, which measures pressure differential in inches of water. By the time I got the needle to move on the gauge, I could not see any of the screen on the filter due to dirt. Even then, it was still less than .5" of water pressure difference.