Per BMW, these fluids are life-long, but I would like to change them once in a while...
4.0 Oil Change Intervals
Maintenance requirements per the vehicle’s Service Booklet or the respective Service Maintenance Checklist.
Replacement final drives: same as for new vehicles.
All models as of 9/97 production are factory-filled with synthetic rear axle oil having a long-life service rating.
No oil changes are required for the life of the vehicle.
E92 M3 DCT