My analysis of your findings would be that you have been gradually cleaning out all the accumulated metal chips in your sump from day one of your engine up to the first oil fill after you installed your magnetic plug. It makes sense that you will catch the most on your first oil change, and gradually less and less until you are seeing only the current crop of new chips on each oil change. It has nothing to do with the weight of the oil. I would stay with the 20W50 weight on an air cooled engine.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville