Here is what I have as far as TEST DATA on K&N's. Nevermind the slick advertising and promotions and scores of users who have never TESTED them.
We have two diesel trucks and heard how much better the K&N's flow. We bought two and put one in each truck. In the course of 6 months the oil analysis ( http://www.blackstone-labs.com/
) showed the silicon level in the used oil jumped from a baseline of 4-5 PPM ( 9 PPM is max allowed ) to 35PPM in each truck. Silicon is the amount of dirt/sand in the oil. Dirt/sand in the oil = BAD.
We switched back to OEM air filters and the silicon level came back to 5PPM on the next change. We always monitor the oil condition after a change to the engine, injectors, fueling etc.
Others in the truck group have noticed the same results.
Look here: http://home.usadatanet.net/~jbplock/ISO5011/SPICER.htm
Notice on the third graph from the top, "accumulative gain - total dirt passed", look how the vaunted K&N performed. The K&N passed 7grs of dirt, the OEM filter passed 0.4 grs of dirt.
Scores of K&N users will claim that they have not had any problems, of course not, abrasive damage is a slow process of accelerated wear.