100 different people will give you 100 different answers and all swear the other 99 guys are dead wrong.
I prefer Shell Rotella T 15w40 in my transmissions.. Hell, I like the stuff in my truck too.. It's a good durable oil. It doesn't have a lot of the friction modifiers that you'll find in typical automotive oil, so it's not going to make your clutch slip. It's also cheap and easy to find.
I also heard that some of the pro race teams use it in their race bikes. Just a rumor, so take that for what it's worth.
I've heard of people running ATF in their gear box.. I've even heard of people buying actual motorcycle oil.. Strange huh?