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
that auction site sells manuals once in a while. if you visit the motorcycle shop you'll see several oils that specifically say for the bottom end of a two stroke. you'll be safe with them. the amount should be stamped on the clutch cover near the filler hole. don't really need to buy the manual for that. helpful for many other things though. or break down and buy a service manual.