Don't use petrol, it breaks down the foam! I use a product called no-toil on my dirt bikes, it works really well. Or use methylated spirits to break down the oil, then wash with warm soapy water. Hang out to dry (not in the sun, the UV will attack the foam too) then re-oil with your favorite air-filter oil and your good to go!

http://www.notoil.com/prod.html

Here's the link to no-toil. Its also "green" if your worried about carbons and two strokes environmental impacts??