I use foam filters......
When I was guiding in Morocco we cleaned the filters every evening after a full days ride. (Honda CRF 450's)
Cleaned in petrol,dried then oiled with spray on Castrol filter oil. I also smeared grease around where the filter seats against the air intake thus providing a perfect seal.
As an aside when at home here,I use the above method of cleaning my Gas Gas air filters but with...2 rinses in petrol,and another 2 washes/rinses in hot water using washing machine detergent. There'll always be more grit after the 2 petrol rinses. Then rinse out in cold water and hung to dry. I have 3 filters that I always do at the same time.
If you've no air filter oil I'd use chain saw chain oil,EP90 or after that normal engine oil...anything as long as it's wet!!!... and if you've no grease spray more on the bit of the filter that seals on the air intake.
(TIP;If you oil the filter and put it in a plastic bag to squeeze the oil through it it will save you having oil all over your hands)
Good advice also ref. hanging back out of the wash of other motos....