I replaced the air filter in my 300D with a K&N but it's in the stock air filter housing. There have been reports that K&N air filters don't trap dirt very well and let larger amounts of dust into your engine. Independent testing from the Air Research Institue has proven these rumors to be false, but ONLY if you oil your filter very well. Lack of filter oil was the reason given for questionable performance oil bath air filters like K&N. Also cleaning and re-oiling much more often was recommended. We have severe dust storms here in Arizona and I'm satisfied my K&N is doing it's job, but I clean and re-lube at every oil change. 50,000 miles between cleaning and re-oiling in my opinion isn't very realistic. A cold air tube with an aftermarket filter won't jive with certain emission control requirements in various parts of the country, like here in Phoenix, for instance. But a Oil bath air filter placed into the factory housing passes just fine. i can't get a straight answer as to their reasoning!