A service manual will tell you the reccomended oil to use but there are many other oils out there specificly designed for the transmission/case/clutch such as gear saver. there are probably hundreds of different oils ranging from a few bucks to about 15 a quart. I use gear saver its about 8 bucks a quart and change it every other ride (this depends on how long you ride and how hard you ride) again a service manual will tell you the reccomended change intervals. Some bikes use regular motor oils and some people prefer specialty oils. It's up to you.
I really dont recommend the use of pre-mix oil for a few reasons *I'ts very thin *expensive * and not designed for gears or a clutch as far as I know (which may not be that much :coocoo: )
now just wait for everyone else to come in here and bash my gear saver oil. LOL
But just wait a little bit and see what others have to say