For the last 18 years I have used the same "Bel-Ray Hypoid Gear Oil" in the 80W90 grade - both in the transmission and the rear-drive. This Bel-Ray gear oil is RED and meets the requirements /specs specified in the K1200RS BMW shop manual and owner's guide (for both transmission and rear-drive). I never had a leak so had there was no need to try different color gear oil in both zones / areas.
Bought my K1200RS new in 2002 and except for the 1st inspection at 600 miles (1000 KM) done at dealer, after this for the last 18 years I have done all the maintenance myself. Since mid 2002, my K1200RS has never seen a dealer (except bringing them the wheels to mount tires in some occasion).
Depending on mileage done during a year (varies from 4,000 to 10,000 miles), I would change transmission oil about every 2 years and rear-drive oil every year. This tigher intervall for rear-drive was mainly a visual monitoring mechanism for metallic deposits in rear-drive. My K1200RS now has 97,000 miles (157,000 KM).
Although I have seen a few problematic rear-drives on K1200RS/GT and K1200LT (friends), mine has been fine all these years. Anectodal evidences from many large BMW user groups (and meeting at rallyes), shows that having a bad one is a crap shoot - many are good some are bad... The shimming and built tolerances of all parts is critical (amost an art) and the factory has not always been on top of things for the quality control.
Jean, your 'Bio' above helps explain why I've found your posts so informative and useful, since I joined this forum.
Thanks for your thoughts, again. When I drain the Final Drive oil to replace it with the Bel-Ray, I'll be looking for the amount of metal deposits in the oil and will snap and post a pic. My local dealership mechanic told me that the metal particles can make the pinion bearing wear, which is what eventually causes the seal to leak. So there may be some correlation between how heavy the metal loading is and whether a particular FD pinion seal leaks.
Because I've owned this 16-yr-old bike for only a year, unlike you I don't know the prior history of maintenance and/or abuse of this FD unit...