Typically, high flow air filters are engineered to trap dirt based upon a longer, more complex pathway for a particle to get through the bed and less upon the bed pore size. The idea is that if a micro-particle impacts the filter bed, it will stick and go no further. Oiled filters substantially enhance this stickiness and become somewhat more effective as they load up with dirt, to a point that they must be cleaned. The pre-filters actually prevent larger material (grass, etc.) from reaching the filter bed, but allow smaller particles (and air) through. I run oiled K&Ns on all of my vehicles. My Fusion hybrid tells me when its time to clean it by a 5-8 mpg drop in fuel efficiency that's restored upon cleaning. (A Fusion hybrid should consistently provide right about 40 mpg if maintained and driven correctly.)
