You really don't want to run 15w-50 or 20w-50 in a tight toleranced, high revving engine (forget motorcycles, we're talking cars here). The Mobil 1, 5w-30 and 10w-30 have shown excellent results in this motor. I don't see a need for anything thicker.
I plan on running the Amsoil 0w-30 in my 225 hp Audi TT quattro. I may try some of their other grades just to generate some comparative data. However I see no reason to go with a 40wt or 50wt synthetic in almost any new engine, except perhaps the "M" series BMW's or something that is aircooled and runs extremely hot.
TooSlick
Dixie Synthetics