So, I rescued a 1986 Interstate. I did a stack of stuff and found a good, willing motorcycle hiding behind years of neglect. After a seasons riding I noticed the drive shaft boot was open at the bottom. No problem, just snapped it back on. Prior to the next season I checked the final drive oil level and curiously it was higher than it should be.
At this stage a big flashing light bulb should have come on in my head, but it didn't.
Early this season I was flying along the highway full of the joys of spring when a terrible clattering and loss of drive stranded me at the side of the road. What I discovered was the driveshaft to final drive spline had disintegrated, both male and female were an arid mess of rust dust and blue metal chips.
Now I understand both what should happen and what did happen.
The oil in the final drive is supposed to make its way through two holes in the bottom of the pinion drive cup and lubricate the lower driveshaft spline. The oil is contained by an oil seal.
In my case the boot leaked water down the shaft and either wrecked the seal or it was already damaged. The water found its way into the final drive and raised the oil level. The holes in the pinion cup rusted over and I was on borrowed time until friction destroyed the joint.
The fix is underway but a couple of things occur to me which might help others.
Do not pack the lower driveshaft joint with grease, it will block the vital oil holes. A light smear is all that is needed. Do replace the oil seal whenever disassembled, it does more than keep the oil in. The boot is more important than you think. It must be watertight.
If an oil level goes up - WAKE UP!