k&N air filter HAHA!

by the way, the best way to clean air filters is to use mineral turpentine (a type of paint thinner that can be purchased at harware stores). HPI recommends using nitro fuel, but that is expensive compared to mineral turpentine.

And NO YOU WON'T DAMAGE THE FOAM. I , along with every one else in the dirt bike world have been using mineral turpentine with 100% success.
turpentine is a solvent of air filter oil, so if some turpentine remains in the filter it will just mix with the oil (you don't want much residue though since it will thin the oil too much).

using water and detergent is a bad idea, since water almost always remains in the foam....not good. plus it is an ineffective cleaner compared to turps.
never use gasoline, since it will make the foam swell and break down over time.

i heard on other forum that people are so paranoid about their "expensive" nitro engines that they put a brand new filter on every time.
My dirt bike engine is worth thousands of dollars alone, but i clean my air cleaners since the foam can last around 30-50 or so washes before falling apart.