[Superman]

I have a friend who runs a fleet maintenance shop. Pretty high volume. Lots of cars, and LOTS of heavy equipment. He has reviewed the research and is completely convinced that dirty air filters protect engines better than clean ones. Clean ones let larger particles pass through. Dirty ones catch more of the larger particles. Air filters have so much surface area that they can be mostly plugged with dust, and still allow plenty of air though for good engine operation. They keep a close eye on engine operation, intake vacuums, yadda yadda. Dirty filters are better.