The clutch input spline was opened up today on my new-to-me -87 K75S. It was very dry but showed very little, if any, wear. The middle spline was also very dry but in great shape. The RD spline had some lube and was also in very good condition.
I assume the RD was lubed at least once before. Who knows. I have no record of it. The Service History book has a BMW Inspection II on 7-7-94 at 10,023 miles. I'm going to assume that that is the last time the clutch/input shaft was done. The bike currently has 21,000 miles. I see that the Service History book says:
Clutch splines / Input shaft
This vital area must be lubricated once a year. If vehicle is used in costal areas or other high humidity areas this service must be performed bi-anually.
It'll be done now though and I'm going to call it good probably for the length of my ownership as I don't put a whole bunch of miles on my motorcycle. I'll continue to do the RD (and other end of the DS) with tire changes. FWIW, in my 300 miles of riding the bike, I did experience downshifting difficulties numerous times. Hopefully that will be cured!