The transfer case isnt just 75W, its actually a highly friction modified ATF type oil for the clutch pack with extra EP additives for the chain drive.
Its shell TF0870, which is now obsolete and has been upgraded to BMW DTF1 fluid. Which basically BMW buys from Shell. I used to fill up with rockoil gearaxl TF0870 as I found a case of it very cheap during covid lockdown and in regular days I can buy it from any brand dealer - even Hyundai have it as a part number for some of their AWD cars.
Nissan also sell it as "GTR special gear oil" and also as Nissan center diff oil for their titans, armadas and QX80s too. But apparently nissan think that people are dumbasses and will pay 100 dollars a litre for it. LOL!