After a while, I'd replace foams, but that would get very expensive very quick if done for every change.

I use Motul foam air filter oil. It can be picked up from most motorcycle shops that carry the Motul brand of lubricants. 1 Liter goes for $15 or so. It goes on very thin, almost like water, but dries to a nice low tack feel. I also like that it's a very bright blue; that helps to identify any spots on the filter that need more oil.