10w30 should cover just about all temps you'll be riding in. If you're riding in below freezing temps, 5w30 might be better.
You can use 10w40, 15w40, and 20w50, too. Regular or synthetic doesn't really matter, as long as you have oil!
The manual calls for straight 30 weight oil, but IMO, that oil is inappropriate for us. Straight 30 is designed for long oil change intervals
(it doesn't break down as quickly as multi-viscocity oil) and it's quite thick at lower temps when the engine is cold.