I don't think there are alternative transfer case fluids for the x-drive models. Dexron works fine in the non x-drive versions, but the fluid friction characteristics will be specific with the BMW fluid designed for the x-drive case, since there are clutch plates.

It is also important to reset the adaptations after replacing the fluid in an x-drive transfer case. You need a BMW service computer for that. The adaptations interpret how much clutch pressure is required in the transfer case, depending on fluid and clutch plate condition.

IMO, a mistake in either of the above could conceivably cause a problem with the actuator motor, since the vehicle computer is continually controlling he actuator, to modify transfer case clutch pressure, in order to vary the driveline torque.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue