I think 99% of all motor oil contaminants are the byproduct of combustion and driving style, and are created from 'within'. Operating in a dusty or dirty environment might affect the oil cleanliness, but not to a great degree...air cleaner or otherwise. Premature cylinder wear is usually associated with poor incoming air filtration.
Not too many bugs make it through the firestorm inside the cylinder to be deposited into the crankcase. Does this sound reasonable!
Stuart
