I might have missed which bike you got. The owners h/b for 660R/X says 10w-30(-10~30+ deg) or 10w-40 (-10~40+deg) or 20w-50 for higher temp.
If you are doing a lot of low speed riding may be a good idea going for a 10-20w-50. It doesn't say which type. Just get a good quality oil. As for me, I use full synthetic.
You may already know this but the 660 has a dry sump so checking oil level method is different: go for a ride, park the bike dead vertical (or as close to it as you can) then check the oil (careful cause it is hot).
Some reckon the oil will stay in the frame, some say it will slowly migrates back into the engine hence giving you an incorrect reading!
Yamaha dealers in cirencester or swindon can help you out if you really get stuck