You can go with Synthetic blends but not 100% pure synthetic unless you bike is a post 1980 year pike. It seems that 1980 was the cut off year for seals to work with synthetic oil. But then I've heard of people with later bikes having trouble. Perhaps it took a while to get rid of all those pre-1980 seals.
Since I have a March of 1980 bike....

On a cost benefit I'm beginning to think Valvoline Vr1 20W50 in the summer and Valvoline 10W40 motorcycle oil in the winter. I'm currently using Spectro 20W50 and 10W40 but at $10.00+ and becoming increasingly hard to get at a case price I think I'm about to give up on Spectro.
I currently use the Valvoline Synthetic blend in the drive train and have been for years: 85W140 summer and 80W90 in the winter (dark blue bottle) final drive , drive shaft and tranny.
Read the Snowbum Article: +1. Wish he would convert % by weight to ppm. Kind of an apples and oranges thing.