Google Spicer diesel filter report. You'll have lab grade comparison of how good or how bad K&N filters work, or don't work, at keeping dirt out of your engine.
Readers Digest condensed version: Not one engine manufacturer uses them from the factory, on any of their equipment. Any motor that needs to really filter, in industrial on critical conditions like mining or agriculture with a limited window or time for harvesting when ripe, they ALL run paper air filter elements for final filtration.
There's your sign.