Cars typically use a paper air filter. Small engines vary: for example my emergency power generator uses a single foam air filter element that is supposed to be oiled, but my lawnmower has a paper air filter element. However, an older version of the same lawnmower I have had two foam air filter elements, one of which is supposed to be oiled.
The two different technologies surely have their own benefits and drawbacks. What are those? When is a paper air filter element better and when is a foam element better?
I understand that cleaning thoroughly paper air filter element is not possible, so if tapping the element doesn't help, the element should be replaced (and finding a compatible part for a 20 year old small engine probably isn't possible). On the other hand, a foam element is difficult to clean (since you need detergent and oil, and you need to dispose of the cleaning water somehow in an environmentally friendly manner) but probably can be cleaned a larger number of times. Are there other benefits and drawbacks?