Cillit Bang!
No really.
Screwd.
PS No not really. I find it unlikely in the extreme that any filter component is going to have a problem with petrol. There is probably no "right" answer for every type of foam filter. You'll need to see the mfr original handbook if it's an original element, else look up the particular replacement.
If in the destructions it says "Do Not Use x to Clean this Filter", then don't...
It's not necessary to tell me I'm right.
Ever. We'll assume it.
Nick Nolte, The Thin Red Line