Dude. ALWAYS lube splines when you have that shizzle apart, and then every 30k to 40k miles. If you put the grease on the input spline only and liberally, it will do the job nicely AND NOT fling on to the engine side of the clutch disk, as it might were you to lube the clutch disk splines and have it be pushed into the forbidden zone by the shaft upon insertion. I use Guard Dog Moly.
Ask yourself, would you rather replace your input shaft, which is BIG $, and good luck finding someone to do it competently, or would you rather ride in sound assurance that all will remain intact and shift as smoothly as the agricultural nature of BMW transmissions of that era will do.