Here's how it is on that topic:
The pinion cup and shaft splines are, of course, highly stressed and need full time lubrication.
You can opt for either of the two debated options:
1. Pack the cup with grease and moly lube, and keep that shaft seal replaced and fresh every few years.
2. Employ gear oil lubrication: Make sure those holes you will see going between the rear drive and pinion cup are kept open, and not clogged with heavy grease. If you look close you'll see that those holes are configured to produce some pumping action to keep the pinion cup lubricated at all times. Grease is only needed at initial assembly, and not much is needed.
Both ways will keep the pinion splines in good shape. You gotta keep that seal fresh, and pressed down to where it seals, and if using grease, you gotta keep that fresh, too. Using the gear oil method you just have to keep the seal fresh, and in good shape.
I use the gear oil lube, and when I pull the shaft out of the cup at every tire change, its bathed in gear oil, no wear is seen.
Some will claim that the holes don't connect the pinion cup to the gear oil lube in the rear drive, based on a drawing view in the manual. But I can tell you as someone who's used CAD programs that this is only due to the draftsman failing to rotate the gear so the holes lined up before taking the cross section view. It was done with a solid model, like Solidworks, which method has given rise to huge improvements in the quality of many, many items, motorcycles among them.
Also, you need to grease the splines on the other end of the shaft and at the other side of the u-joint, too.
I was surprised when replacing the shaft on mine (yes, I learned the hard way) that the new shaft, pinion cup and seal together were cheaper than a new motorcycle chain, as used on my other bike, which need to be replaced about every 20,000 miles or so! They're more expensive, but a lot easier to replace, a trade-off, like everything in this world.
I found the 2 holes flowed oil from the differential when I had it out on the bench, stood the drive on the pinion cup and oil ran all over my bench!
Just my $.01875.
Hay

Jimmyt