I use the Amsoil oil wetted foam filter in my car, but it is some trouble to service once a year. No, I don't sell the Amsoil products, and I don't care for multi-level marketing organiztions, but the porducts are excellent. The other three cars are using Mann paper filters. I think most paper filters would be okay. The surface area of the filter used in the Volvos is so large, I feel there is little performance to gain with the K&N, and I don't think they do much but keep the big stuff. It is easiest to use the paper filters instead of cleaning the oil wetted foam fitler anually.

Oil wetted foam filters have shown excellent dirt entrapment qualities. Aircraft using them (Brackett brand) have shown a significant reduction in dirt in the engine oil. Many aircraft came from the factory with the oil wetted gauze like the K&N filter. Many dirt bikes used to use the oil wetted foam filters, and they did an excellent job of keeping the dirt out in really nasty conditions. Paper filters become more and more efficient at filtering dirt as they plug up (unless they have a mechanical failure) until they eventually don't flow enough air.