Oiled air filters collect dirt in the oil. There is A LOT of dirt off-road. They are too difficult to clean on the trail. Just a bad idea. Bring a couple paper ones and spend 2 minutes swapping.
I normally swap every 1 to 2 days, depending on time spent in someone's dust. That is tough to do with an oiled filter. Most people don't like throwing away their $60+ filter.
Also, they let more air through...more air means more dirt also comes through, which is probably fine on the street, but definitely not good off-road