final drive fluid for 087

[Trevor Irwin]

I'm putting a used final drive/tranny in a 3-speed 1990 100. replacing
seals and so forth. anyway, the question i have is about what fluid to use
in the final drive. The Bentley says hypoid tranmission oil GL5 (SAE 90
Mil-L 2105 B) (but I am always rather suspicious about just how
accurate the Bentley is, especially since it claims that no refill is
required which we all know  is false). The archives didn't seem to have
anything about what people were using, the dealer doesn't know anything
about my car, but they offered me some Penzoil 80W-90 (non-synth) (the
main selling point with the Penzoil seemed to be that it already had
limited-slip differential additive in it--how necessary is this?). I have
seen Mobile 1 75W-90 synthetic in town. Any suggestions? Thanks.

Trevor