We use Motul 5100 4t 10W40 synthetic in both our Vulcans and we love it, with no issues whatever. I work at a bike shop and so I can get oil cheap and prefer to run the best I can afford.
One claim I heard was if you don't run Synthetic from day one in the bike during break in, then running a synthetic after a convention oil, is just wasting money on expensive oil since the bike was not broken in with synthetic. Can't testify to the truth of it, but I have heard from some knowledgable people on this one.
Current bikes: 2005 Vulcan 1500 Classic