This is a really frustrating business. On my 79 the oil transfers itself quite quickly into the gearbox (transmission). I keep tipping the stuff into the shaft and at the same time draining a surplus from the box. Years ago I had a 71 R60/5, that also quickly lost the shaft oil but it moved into the rear drive. That was a real pain because the surplus dripped on to the rear tyre. In the end I mostly left the shaft alone. Could not have been so bad because in 73 I rode the bike from Belgium to central Turkey and back without refilling the shaft, caused no noise or any other problem. I've studied the diagram and see the joint concerned must have very little movement (it's not a UJ judging by the drawing) so maybe just does not need much lubrication. Perhaps the occasional top-up is enough! What do others think?
You need to replace either the output seal on the transmission which is a big job as you have to remove the output flange - and don't forget that the seal goes in "backwards" (reverse?) of how one would typically think of installing one. And there is a seal on the driveshaft end of the pinion gear in the final drive housing.
If they are leaking, they need to be replaced!
Btw in the US you say transmission for what we call gearbox - for us "transmission" means everything between engine and driven wheels - clutch, gearbox, shaft, bevels, differential if any...so what do you call all that in the US?
Yes, we use the term "transmission" as a noun (i.e. gearbox) which is a pretty sloppy way to treat the Queen's Engrish (

), I agree.
Properly used, "transmission" is a verb, to indicate movement, I believe. (Electrical transmission lines.)
And gearbox is a lot faster to type!
I also have a problem with calling the engine a "motor". To me, if it is electric, it is a motor; if it is mechanical, it is an engine.
But I suppose anything that moves something could be a motor, as in "to motivate".