i noticed that when i ran m1 (red-cap) in my 2002 kawasaki vulcan 800cc's it ran warmer than when i ran amsoil 10w40. seems like to me that the 10w40 let the engine run cooler. i also noticed that the amsoil 10w40 let the bike shift alot smoother than the m1 did. soi changed the m1 out at 500 miles and switched back to amsoil. **** i run 10w40 in my kawasaki vulcan 2000. and its 2053cc's and i still use 10w40. the kaw owners manual recommends 10w40 for a engine of this size, so i would guess to say 10w40 is plenty. i never felt a need to run 50weight in any of my metric engines.
http://www.cruisercustomizing.com/arkainzeye
http://www.cruisercustomizing.com/arkainzeye