They all do not filter oil. Ones like fram have bypass holes that allows the oil to flow around the filter media. Others have bypass valves in the bottom, that open at a whopping 10psi like harley filters. All filters have a by pass valve in the bottom that should open in the even of catastrophic filter media clogging to allow oil flow to the engine. all have anti drain back diaphrams too. some are made of paper. Fram filters have 63 square inches of filter media. Drag, K&N, and S&S, have 165. The larger the surface area the more oil that can flow. Thus the pores that allow unfiltered oil flow around the media. These holes allow huge particles to get through, about the size of the end of a needle. Dont take my word cut one open thats what I did.
