Frankly, I use 10w-30 motor oil, no particular brand, although I like to buy synthetics. Yamaha recommends Yamalube 4 because it is their brand, but they don't say it
must be their brand. Sure, there are alot of expensive brands out there, but when was the last time someone complained about their transmission gears and bearings wearing out? Keep oil in the transmission and change it occassionally, that's the key.
As for the 85w80w gear oil type designations, gear/trans oils are rated differently from motor oils. Here is a nice chart, you can see that 30w is roughly equivalent to 80 or 85 gear oil.
http://www.teamrip.com/viscosity_chart.html
While it's true that gear oils have better extreme pressure additives, Blasters only have like 16-35 HP pushing some straight and helical cut gears. IT'S NOT AN EXTREME APPLICATION.
Honda's HP TRANS will work fine, Belray will work fine, AMSOIL will work fine, etc......If it says it is motorcycle transmission oil and you want to pay $10+ a quart, fine. The only real rule is try not to mix different oils 'cause sometimes they will react with each other and gum up (theoretically at least, I've never actually seen this happen).
If your clutch slips, it's worn out or not adjusted correctly.