Since we are having this discussion about air filters I thought I would share a strictly anecdotal tidbit about one that I just experienced.  My Lincoln Town Car uses a rectangular shaped air filter in the air box... a typical Fram or STP unit costs about $20, a K&N costs $40.  2 oil changes back we replaced the air filter with one of those $20 units.  Less than a month later (about 6k miles, time for another oil change) I noticed that I was starting to hear some detonation when I put my foot in it (4.6L dual O/H cam V8) so I opened up the filter box and found that my 6k mile old filter was really dirty and starting to clog up.  I replaced it with a K&N and found the performance to be greatly improved and the sound of detonation completely absent regardless of how hard I pushed it.  All I know for sure is that I'll never go back to a plain paper filter again in that vehicle and I'll service the K&N at regular intervals far less than what what they recommend, just to be sure.  Anyway, I thought it was interesting that I was hearing this detonation sound (you know, where it sounds like marbles bouncing around under the hood) as a result of diminished airflow through the filter... the engine is controlled by a computer that receives input from the all those sensors, including a mass airflow sensor at the filter box and a throttle position sensor at the throttle body... I thought that all that computer control would obviate the possibility of detonation.  Guess I was wrong.

 <marv>