OK, got ya, o my "98 they recommended 140 and thats what I used in trans and final drive. This '12 model, all I have seen says use the 90wt. thats what I have purchased, but have not changed them yet.
Yes, the 1200 hexhead BMW manual called for Brand name hypoid-gear lubricant SAE 90 API GL5. I was surprised to find the newer camhead manual called for SAF-XO gear oil (that is a 75w90).
We have been using the 75w90 & 75w140 in the hexhead for a long time now (even a lot of BMW dealers do).
The straight 90 weight won't hurt the trans as the gears are pretty robust & will hold up fine with about any GL5 gear oil & most of the trans bearings are sealed so run in their own sealed-in grease.
At hot bike running temps probably not much difference between the straight 90 GL5 & 75w90 GL5 as far as operating viscosity is concerned. The BIG difference is in the cold shifting of the 75wXX over straight 90 weight.
The straight 90 weight is probably a bit less prone to shearing as the 75w90 & 75w140 have some polymers to get that viscosity spread but most of that is probably offset by the base stock quality being better on the up-level 75wXX. (not easy to find a high end good quality straight 90 weight gear oil that uses up-level base stock)