There was a post some time ago with a link to a Ford diesel pick-up site .... wherein some members were able to do their own air-filter tests on machinery designed for that where one of them worked. They bought a selection of filters and went to work -- incl a K&N.

The K&N flowed more air than the rest of them -- but also let more dirt thru. I don't remember all the figures ........... but the the AC Delco filter let .7 gm of dirt thru ........... and the K&N let 7 gm thru. I pulled the K&N off my car, giving up what felt like a (very welcome) couple of horsepower in the process.