The JAMO is a Japanese motorcyle rating. I guess they just expect you to know this when they put the JAMO MA rating on the Rotella T6 meaning it is formulated to be safe for wet clutches in motorcycles.
The oil the dealer puts in is strictly based on either where he gets his oil cheapest or subsidized. Kawa uses Kawa oil, Honda uses Honda oil, etc. Aprilia/Vespa puts Eni Agip 5W40 synthetic oil in EVERYTHING from the factory. The Sportcity 50 cc scooter to the Tuono V4 all get the same oil. I hope this tells you something, namely, that there are lots of good oils out there and opinions are often motivated by salesmen, not science. Basically a synthetic is a better choice than dino oil due to it's resistence to breakdown, and as long as you stay away from the "Energy Conserving" oil which can cause the clutch to slip, you will be fine. Amsoil, Mobil, Shell, Royal Purple, Pennsoil, Motul...it's all good stuff.