.. I am going to get shot down in flames by BMW purists, but so be it.
I use powdered tungsten disulphide, mixed with Mobil GL6 synthetic gear oil in the gearbox and diff.
Before everybody throws their hands in the air, and run screaming from the room, let me explain. Currently in our summer, one of the hottest on record, our daytime temp. has been exceeding 44deg.C. This makes the road temp up in the high 50's to 60's C. (or hotter).
Now, we use mix Tungsten Disulphide with the gear oil, of our farm machinery, and previously, for example, the right angle drive gearbox on our slasher, after being used for 8 hours constantly on a 40deg.C day, got so hot, it melted the little plastic sight glass.
After mixing Tungsten Disulphide with the oil, it runs about 20degC cooler (according to a infared temp. tester).
Since using it in the GS, I have noticed the gearbox and diff, being a little quieter, on extremly hot days.
I am not recomending everybody rush out and add it to their gearbox, but to save from explaining what it does, do a search on google, there are plenty of references as to suppliers, and what it does.
But, in the long run, it is MY bike, and I will do with it what I want.
Regards GT

(ps. DO NOT mix it with engine oil)