UOA - used oil analysis. I was considering the K&N as well until I did some searching on the internet. Seems a couple people have done used oil analysis after running and K&N vs. a stock air filter for a couple thousand miles. The UOA checks for the total number of particulates in the oil, including quantitive data (in PPM) of some of the known bad particles. Their summary is that the K&N's *do not* filter as well as a stock paper filter. They *might* be beneficial for increased air flow, but if engine protection is your main concern you should look elsewhere.