I remember my days as an air cooled VW head, folks who knew and loved these engines never worried a bit about oil filtration (these engines did not have one, just a screen) because the German engineers knew that the oil breather hooked up before the carb would clean the air so thoroughly, there were no thoughts of contamination and/or engine failure. I guess they were correct, as the upright air cooled Vee Dub engine was one of the most successful, trouble free, longest lasting engine designs on the planet. So there must be something said about keeping our air filters clean in the same breath as oil filtration... As Frank Zappa used to say, "...information is not knowledge, knowledge is not intelligence.." and so it goes, boys.