I use a synthetic blend in mine (Valvoline Durablend).
I used 15w40, only because I got a case of it really cheap. Haven't had any problems with it in the Vulcan or my KLX dirtbike.
As for Seafoam in the oil, from what I understood, ya didn't want to leave it in there. Just add some before ya do an oil change, run it a bit, then drain and refill with oil only.
But this is from the Seafoam site and seems like it's OK to leave in the oil....
When Added to Crankcase (Oil)
Frees sticky lifters and rings
Increases R.P.M.'s vacuum and compression
Cleans dirty parts
Cleans PCV valve systems
One pint treats 10 quarts of oil (avg. 1 1/2 ounce per quart).
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
"When life throws you curves,
Aim for the apex."