Automatic transmissions, as our ZF's, are operated and lubricated by a specialized hydraulic fluid. Automatic transmissions have largely advanced beyond shifting bands, using sprag clutches in their stead.

The xDrive transfer case does not have a differential between front and rear drive, but a servo operated multiplate wet clutch. Perhaps all xDrive differentials between left and right wheels have servo operated multiplate wet clutches between the differential and the side, i.e. two per 'axle'. That allows the computers to minutely vary the power to each wheel (power being the product of torque and angle rate).

Lubricants wear and so do their additive packages become exhausted. A good model for a lubricant molecule is a molecule-sized roller-bearing. The atomic bonds within the long chain molecule fatigue and break.

Beyond that, lubrication is a specialized technology. In a luxury car, why quibble over the cost of a fluid replacement?

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