There is a pretty simple rule to follow. A filter that flows more passes more dirt. After doing about 220 TB's now, I can always tell who over cleans their filter(long term)..... The TB will have oily residue, and the throttle shaft will be worn from the excess oil causing grit to cling to moving parts. The best thing you can do is let the K&N stay a little dirty since the dirt will start to act as a filter medium. I worked on dirt track cars for years and have nothing good to say about foam, except we found that it works best on a parts shelf rather than on a car. The best filter medium by far is synthetic, whether it be for air or oil. It is far superior in flow restriction vs dirt passed. I just don't have time to seek the proper ones out for this application until I get some of my other projects finished. The only reasonable option is using the K&N, and not over cleaning it. The dirty filter actually allows less dirt to pass, and has a better flow vs dirt passed ratio than foam. I guess I am telling you guys to stop servicing your ATV to death