This excerpt from a post by Pensioner is the only comment in this thread to
refer to any kind of actual testing (as opposed to marketing hype, theory,
and belief) of the K&N filter. I don't know if the information is accurate
or not, but I think it is worth reposting the quote:

"ASME (mechanical engineering society) did a study years ago for diesel air
filters for earthmoving equipment.  They found the best is a folded treated
paper system FOLLOWED by an oil bath system (a la aircooled bugs) AND
regular maintenance.  They rated the filtration properties of the K&N oiled
foam type as poor.  They were not concerned with HP/MPG only with
preventative maintenance."

Please, I'd love to believe the K&N is both higher perfomance AND better at
filtering out smaller particles than paper filters.... If someone has actual
data to support that PLEASE post it to the list.

Bill Davidson