The diff fluid should be changed at 1,200 miles and then every third oil change. So that would be at 45K miles. I would change it sooner - say at 30K.
Bellow is what my trans fluid looked like after 35K miles(left), new MTF-LT4(for my car) center, and new Redline ATF-D4 far right.
Attachment 726232
I deicded to run Redline fluids bc they just seem to work better than BMW fluids do. But IF your under warranty... you should stick with BMW fluids. Btw... when I switched over to Redline ATF-D4 I noticed my shifter was MUCH smoother. Much like many 1M owners report after they put in the proper MTF-LT-5 trans fluid. You want to use the Castrol SAF-XJ+FM fluid for your diff.
Dack