Airheads need scheduled lubrication on transmission input shaft splines (never grease the clutch disc splines themselves); and, rear wheel cup splines and associated rear drive output splines on the twin-rear-shock models.
Numerous lubricants have been tried in many different climatic and riding conditions, over many years. Some newer lubricants are still being tested. There does not seem to be any magic, perfect lubricant for these places. BMW has specified quite a few lubricants over the years, such as Staburags, Optimoly, ETC. The author has never believed these lubricants were as good as some others, at the times BMW had those recommendations. The author, and others, have done long-term testing on various lubricants. This article you are reading no longer lists these lubricants, nor recommended lubricants, as the latest information will be found HERE:
http://bmwmotorcycletech.info/chemicalsetc.htm where the author keeps the information up-to-date. The spline lubricant information is at item #6A in that article.
Later model (exact year and models are unclear, probably late 1980’s) transmission input splines are SUPPOSEDLY nickel-plated and do not require cleaning and relubrication quite as often, but 30,000 miles seems the practical LIMIT, and for earlier ones perhaps 20,000 is the limit. Best you do it before these mileages, at least once, and then, upon inspection, adjust the interval for the next clean/lube. Once a spline shows rusting, you are LOSING METAL! The transmission input splines are fine-pitch and not very deep….you do NOT want them to fail!