You want to use a synthetic or semi synthetic oil in the 10W-40 or 15W-50 range (Hotter weather, use the higher number). 20W-50 will also work. Synthetic oils are used because they do not loose their viscosity over time in harsh environments like regular oil. A 15W-50 regular non synthetic oil will rapidly become straight 15 weight because of the gears in the transmission which rapidly chop up the viscosity enhancers. This lowered viscosity will no longer be able to prevent metal to metal contact in your bearings eventually causing main or con rod failure. Viscosity enhancers have the ability to increase the viscosity of the oil as it warms up and the reverse as it cools helping with cold weather starts. These enhancers become chopped up in the gear train of the motorcycle. Synthetic oils do not need enhancers. Very simply put (Because I don't know the full story) synthetics can change their viscosity through chemical action triggered by temperature giving better coverage over a higher temp range that air cooled engine typically goes through. They cost more but are needed. If your engine had a separate gear case like the Harleys, you could use a good quality dino multi grade oil with little fear of viscosity breakdown. Because the gearcase and engine are combined, you need some form of synthetics to keep your engine alive.
PS - Synthetics last a lot longer in your engine than regular oil. Probably the only reason for changing synthetic oil is because it becomes contaminated with carbon.
