Specifically labeled M/C oil is probably designed for air cooled engines which are subject to fast changing extremes in temperatures and tolerances. We used to use Aeroshell in our old Harleys--regular auto oil seeped out all over and burned up quick. (Newer bikes with better technolgy don't suffer this problem as much any more.)
By the way--hello, my first post here. I wanted to respond to this, so I finally joined up.