Gary writes, "However, lots of people run filters that quite a bit below the filtering efficiency of a standard paper element (oiled foam or K&N) and I don't know of any widespread problem."

 

All this talk about "paper" air filters is making me nervous.  Certainly some paper air filters are approved and appropriate for our planes, but some are not!  Decades ago I read an article (no reference available) regarding an aviation incident wherein an engine was starved of power because the automotive paper air filter being used had softened and collapsed from rainwater (and high air flow velocities) in flight.  I remember the article well because it was perfectly timed to coincide with my imminent installation of a paper filter (automotive)  into my Lancair.  Scared that I was about to make a big mistake, I wetted that automotive filter in a bucket of water and attempted to breath through it.  I couldn't!  When I came to, I bought and tested an oiled K&N filter.  Breathing through it was easy even when very wet causing me to think too that it might not ice up in freezing conditions as easily as would a standard paper filter.

 

There is paper and then there is aviation certified paper.  Be careful!    Greg Nelson