I went to synthetic in all 3 oil compartments at 4000 miles. I use Silkolene products. It is expensive stuff, but I get a special break on it due to my maintenance agreement. They make an 80-90 wt synthetic gear oil. I would not use synthetic anywhere in the 650 until it has at least 3000 miles on it. You do want to make sure everything has worn in properly. That is more crucial with the engine than with the transmission or final drive, but there is no good reason to move to a synthetic any earlier in the drivetrain. With my motorcycle, synthetic oil made a big difference in the smoothness of shifting the manual transmission. With the scooter, you will feel no difference.