any reason not to just use 10-40 Rich?
In the case of Roltella T it doesn't appear that they make one in that weight, but any good quality automotive 10w40 oil will likely work fine.
Suzuki transmissions are strange ducks, they all seem to shift a little different , and what oil will make them shift best ends up being a trial and error exercise. The best advice I can give is buy good quality oil and experiment a bit. None of the good stuff whether it's automotive oil or an M/C specific gear oil will hurt the trans, but some will shift better than others.