Motorcycle Repair: leaky transmission, meantime thanks, sky is the limit

Question
My shovelhead (1984 FXE 4 speed, 45K miles, never taken apart) transmission gets rid of a pint of RevTech SAE 80W90 trans oil every hundred miles, in daily use. It doesn't drip under the trans when parked, and seems normal otherwise (clunks going into first and wines in fourth) and shifts ok. I can't tell which side is leaking, as there's always a little oil on the bottom of the engine and trans.It only seems to be leaking while in motion. I love wrenchin' and intend to fix this myself as always, but need some direction. Also, how many miles should this trans last? Should I consider a rebuild, or will a new seal last me awhile? Is there a better oil to use in the meantime?             Thanks Mike

Answer
Sounds like your main seal on the trans is leaking. It is located on the chain side of the bike. the inner primary will have to be removed to get to it. The oil you are using is fine. There isnt any oil that will not leak.
The clunking and the whining is pretty normal for the 4 speeds.
As to the miles they last. the sky is the limit. It depends on how you ride. If you slam gears alot, it wont last as long as normal, everyday riding. harley trans are pretty tough and usually have problems with seals.
Good luck and happy holidays
Mike