Guys, I was following some air filter threads over on the Corvette Forum; thought you might find some of the discussion interesting.

One of the guys there made a pretty good case for a reasonably dirty filter not really hurting gas mileage (under normal light-throttle driving conditions).
Seems the pressure drop behind the mostly closed throttle valve is MUCH greater than the pressure drop thru the filter, so no energy wasted getting
the air thru the filter at normal 1/8 to 1/4 throttle openings; even a dirty filter can easily flow enough air to cruise on. And today's engines adjust the fuel mixture to compensate for any slight pressure drop at the filter. Of course, full throttle is another story, and reason enough for Vette guys to change filters often.

Another thread had a big scientific test of a bunch of filter types, including the famous K&N. Bottom line was that the K&N cleaned 97%-plus of the particulates (size I can't remember) but the AC paper filter cleaned 99-plus.
The K&N allowed 17 times as many particulates to pass through; but of course had less restriction to flow at full throttle.

So, I will still change my paper MB filter at 10-15K miles, even though it still looks pretty good. even 1/10 MPG improvement pays for a filter pretty quick.

DG