To answer question #1,
I doubt anyone can show me a 15/40, 20/50 or Mystik 15/50 oil that sheared down to a 20 weight, with reasonable mileage of 5000 miles, in a motorcycle.
I can.... I did some oil testing for a blending company that will remain.... nameless. Their 15w50 synthetic sheared to a very high 20wt in 1,700 miles of my abuse.
In a shared sump situation with nasty off-road riding, where there is a lot of steep climbs and a ton of clutch slipping.... Oil is destroyed. ....and I say any oil.
The company was so upset with the results, I suggested they pick a couple of top-shelf motorcycle oil brands and let me take them through the paces and test them as they did their own..... Yep, same results.
I contend there is a world of difference in motorcycle oil performance, when you compare city riding (hundreds of stop lights and stop and go...), compared to someone that either commutes many miles with few stops, or the rider that climbs on a bike and runs down an interstate for hundreds of miles.
A lot of clutch work and the oil will take a pounding.
In my experience, 15w40 HDEO with JASO certs is about the best there is.