Well:
Multivis 5W40 wont gum up in the winter so bad.
Hypoid 85W90 may get quite thick in the winter.
Transmission Fluid probably too thin to provide good coverage and non-compressability is not an advantageous property.

When the gears turn, as there is no actual pump, the oil must be thick enough to adhere to the gears so that all round lubrication coverage is achieved. I don't think Tranny fluid is optimum for this application.

Just my opinion.

The oil that came out of my diff was about the viscosity of 30W oil so I put in Multivis 5W40 oil. There is enough oil mass in the diff if it is filled to overflowing at the fill port to handle most lubrication needs. No weird noises from the rear end.

Years ago, I had a Toyota transmission rebuilt. The rebuilders put hypoid in it when the manual called for 30W. The transmission ATE itself just after the warranty expired.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)