I wonder if it's wear on the pressure side of oil pump that allows some to wet sump more than others?
A check valve would be the best solution but you have to be sure of its reliability. My Velocette Venom had such employing a spring loaded inverted ball valve at the bottom of oil tank, relying on suction from oil pump to open valve against light spring. Important that feed line didn't contain air though. I recall an owner retro-fitting similar from a Japanese bike when his OE valve seat became worn.
