Syn oil is fine to run in the tranny. But gear oil is better. I switched to Mobil 1 gear oil in the tranny and noticed a huge difference in ease of shifting.

 

How long have you been running gear oil, and does it make your clutch slip at all?

 

I have about 15,000 miles on the bike since switching to gear oil. No problems.

cHarley can give ya all the low down on the technical stuff between gear oil and motor oil. It's 75 w 90 weight gear oil. At least I think those are the numbers. Sounds like it would be thick, but there is something about the way they assign the numbers in gear oil differently than with motor oil.

All I know is that the tranny seems a LOT more happy with the gear oil. (Man, does that stuff stink though!!)