k&n air filter cleaner,they sell it in refillable containers.i spray it on ,let set 10 minutes,the rinse out the junk,it says it biodegradeble too.
+1 just started using k&n on my foam air filters too and it works great. To store your guys pre-oiled filters, you guys leave them in a sealed plastic bag right? I bought a preoiled twin air filter and the bag had breathing holes in it allowing in air. Is that normal? I figure our air filters in our bikes are exposed but all the other no toil pre oiled filters I bought had completely sealed bags