I have tried any and all remedies to cure the wet sumping on my M21. The problem is I have over 30 bikes and can't ride them all. Also where I live it can get hot in the summer (111 degrees a week ago) which makes the wet sumping worse. My Norton Commando will also empty the oil tank in the summer. The only old bike I have which does not do this in my Velocette which has from the factory a check valve in the oil line.
I finally this year fitted a shut off valve to the M21 which has cured the problem. Yes I know I could forget to turn on the oil but am not worried as on all my old bikes they get a full inspection before starting and any time I stop for fuel, food or beer. I always check to see if oil is going in to the motor and returning to the tank before the bike moves.
