Carl,
You are probably correct regarding junk/debris (airborne dust/pollen/dirt) getting sucked into 1955st and earlier valve covers. It would get suspended in detergent oil
If detergent oil is used, and the engine has an oil filter (by-pass or full-flow), that "debris" would stay in suspension (and, get filtered out, if a filter is used). Change the oil every 3000 miles and nothing should drop out into the pan?
