Quoted by 1tuffram:
I wondered this very same question myself after reading the tread "K&N dosn't stop dirt" in the products forum [how the paper and K&N filters compare]. I contacted both K&N and Amsoil about the comparison of this two filters. I got two different answers, so you you decide.
K&N response was "Our air filters are tested by 2 outside, independent labs. They have been proven to stop at least 97% of particles on a SAE fine dust test. This test uses particles mostly in the 0 - 5 micron range but goes up to 20 microns. For comparison, a paper filter stops 98% on the same test and the OEM minimun standard is 95% Foam (Amsoil is foam) is generally the worst media with a typical efficiency rating of 75%. To get higher ratings, the foam must be more dense and therefore way more restrictive".
Next I contacted Amsoil with K&N's response, this is what Amsoil said in their defense. "The only way K&N can get their effeciency rating up to 97% is by oiling it for "off road" use, which reduces air flow. Information published by Amsoil is done by an independent lab, therefore certified correct. Tests showed Amsoil effeciency at 99. 5% and the K&N at 94. 9%. While the K&N with normal oil levels had a slightly better new filter air flow than the Amsoil, the Off Road oiled K&N was worse. In addition, the K&N plugged up with dirt much quicker lasting only 10 minutes before its resistance to flow was 5" water. Amsoil lasted 37. 5 minutes in this same test. The Amsoil filter works because the foam filter oil does the dirt removal, not the media".
There you have it, the response from each company. One other thing you may want to consider is the efforts needed to maintenance these filters. I own both and feel the K&N is easier to clean and reoil, the down side is cleaning all the sealing grease from the airbox lid and off the lip of the filter when servicing the K&N filter (which really is not to big of a deal).
After using both air filters I decided to stay with the Amsoil filter. To each his own. Hope this might be of some help.