Shogs: The splines get buggered because they go dry so the bottom line is that dry splines are the cause and the buggered splines are the result. Usually the clutch plate splines start to go first but as those start to get bad they eventually start eating up the transmission input splines.
The spline lube CAN last, albeit not forever, but that's why I now use the 50/50 Honda/Wurth combo because the Wurth SIG 3000 uber-stickiness keeps the moly around longer.
There is also the VERY mistaken notion about that you need to be sparing with the spline lube because if you use too much it will spin off onto the clutch friction surface. This is only partially true. If the lube comes out of the front of the splines then it can indeed spin off onto the clutch friction surface. However, if it comes out the back of the splines then, due to the raised collar on the rear of the clutch friction plate then it will just spin off harmlessly onto the inside of the bellhousing. So the best approach is to clean all of the old grease out of both sets of splines and lube the hell out of
only the transmission input splines before putting the transmission back on.
It this picture below I used Mobil 1 synthetic bearing grease for demonstration purposes only because it's red and more visible in the picture. It's a clutch plate assembly pushed onto transmission input splines. The very fact that you can see that excess lube from the side demonstrates that it is well behind the rear clutch plate friction surface and will just spin off onto the bellhousing.
And here's how much of the Honda/Wurth 50/50 lube that I used when assembling my K75F:
Also note that in 1990 BMW hardened the transmission input splines a bit so spline lubing is a tad more critical on the early bikes.
