Use the GL5 hypoid lubricant in the rear axle assembly. The GL5 lubricants are not required for the transmissions, and typically are too slippery for proper synchronizer action, as Jerry stated.
My preference - in part based on what I keep in my shop to service several older BMWs - is Red Line Oil MT-90 for these older transmissions. The MT-90 is GL4-rated - sufficient for transmissions - has the correct viscosity, correct slipperiness for synchronizers - which most of the GL5 lubricants do not - and works well in a variety of transmissions since it's yellow-metal friendly. The Red Line Oil MTL (too light in viscosity for the 'Bolts) has quieted more than one noisy or "grunchy" BMW/Getrag transmission. The MT-90 product uses the same technology as MTL in a bit higher viscosity. For an interesting read, here is a
White-paper from Red Line.
There are other brand products that are similar - look for one that is very specifically noted for transmission service if you decide on something besides the Red Line Oil product, or Ford-spec mineral oil.
Hope this helps.
Doug
