On my pipercross panel filter I leave the filter soaking in petrol for a few hours to soften the oil and then using warm water and fairy liquid I massage the crap out of it (quite literally)and keep rinsing and re-adding fairy liquid until no more dirt comes out and the filter is soft. Then I rinse it off with bold water from the inside out to prevent any remaining dirt going into the thin side of the foam. Then let it naturally dry in a warm room and recover with the air filter oil. Personally I use a competition grade oil my dad gave me from his racing days, but it's recommended to use the specific oil for that filter.
