Honestly the K&N (et al) spray stuff is hard to beat for ease of application and a tall can of it will last a fair while. If you only have one bike it could be a justifiable expense if for no reason other than convenience.
But oil is oil when it comes to foam air filters and its only purpose is to remain wet, catch dust, not be so thick as to restrict airflow and not contain ingredients which are destructive to the foam.
I keep a gallon or two of cheap house brand Dexron2 ATF around at all times anyway. I use it to flush forks and as fork oil on the TW and a couple of other bikes, chain lube when I'm on roadtrips, mix it with MEK or acetone to free up stubborn nuts and bolts and a boatload of other stuff. Works on air cleaners as well and keeps me from accumulaing a zillion little aerosol cans of various products and preparations that always seem to go empty at inopertune moments.
But mostly I use it cuz I'm a cheap bastit.
The reason I use Dexron2 is for the fork oil function. They keep reformulating all the other types of tranny oils and their viscosities are always changing. Dexron2 (so far) has remained a consistent 8-10W, depending on the brand.