Well, you will find lots of different answers and preferences but what ever meets the manual's specs are always the best choice. For the engine and transmission (as they are shared), after full break-in most of us go to a synthetic oil as close to the weights shown in the manual for the temps and..most important, they meet the JASO MA rating for the wet clutch. You can use anything from 0w40 to 15w40. It is not recommended to use a heavier oil then the 40. I use the Castrol Power RS or Power1 Racing 4T 10W40 but there are many other good oils out there including some meant for use in diesel engines that people do use.
As for the differential and gear case, as I remember, the book calls for 80W90 or motor oil depending on the temp conditions. I choose to use a synthetic 75W-90 just because the sling starts at a slower speed cold off the ring gear then with the 80. I use Valvoline Synthetic 75W-90 but there are many others out there.