[Aronis]

The G4 oil is synthetic while the G1, G2 and G3 are mineral.

G1 and G2 are equivalent to SAE 80W90
G3 and G4 are equivalent to SAE 75W90.

The oil used in the Differential on the M550ix is Redline Manual Transmission / Differential Fluid 75w90 - Mfg Part # 8352257904, ECS lists same oil for front and rear differential, perhaps someone can chime in and verify this BMW part number.

The product description for this item:

Red Line 75W90 gear oil is a great choice for a fluid service in many differentials and some manual transmissions.
The preferred lube for most car and light truck differentials, 75W90 is also used in many racing applications. Contains a special friction modifier for limited slip differentials; promotes synchro braking for smooth shifting, cold or hot.
Exceeds API GL-5.
Red Line MTL may be used in transmissions which recommend 75W, 80W, or 85W GL-4 gear oils, or SAE 30 or 5W/10W30 motor oils. If a 90W GL-4 or SAE 40, 10W40, or 15W40 is required, MT-90 may be used. If the transmission or transaxle requires an SAE 90 GL-5 gear oil, then Red Line 75W90NS or 75W140NS Gear Oil may be used.

So it seams to me that using the "lesser fluid" used in the M550ix would not be a problem. Now it comes down to some nit picking product brand name???

Mike