On the 05 Rancher and 06 Foreman I owned, I used Wix filters. Available at any auto parts store for just a few bucks. I don't know the part # but its the same for all years of Rancher 350 and Foreman 500 (possibly more models also) Most oil filters are made by one or two companies and just branded by the company selling them (Honda, Wix, Emco,etc.)