Hi, I need to put oil in my AMC gearbox, my Haynes manual says Hypoy but no grade - I presume its 80 or 90. But Mr Google tells me it is SAE 40 or 50. Our technical section only describes oil for the lightweights. Hypoid 80 or 90 seems far too heavy to me but then it does have straight cut gears. If you can help, and I am sure someone can, many thanks.
Hypoid grades 80 and 90 are…
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Oily Chunks
The attached diagram shows the recommended oils for a Dominator. John Hudson used GTX 15W50 in his Mercury. Claiming that it was suitable (but not the best) for all applications. So engine, gearbox, primary drive and forks. The bonus of this multigrade being that, in winter, it did help the engine start more easily.
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Gearbox oil
Whatever oil you choose may I suggest adding some Lucas Oil Additive? It helps the oil cling to the teeth as they rotate. Mark
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Hypoid grades 80 and 90 are roughly the same viscosity as engine oil grades 40 or 50 at gearbox temperatures. Hypoid (extreme pressure) oils are allegedly not needed for straight cut gear boxes. They were designed for hypoid bevel gears in car back axles, where the eccentricity of the input drive puts big sliding movements on the gear contact surfaces. They used to damage bronze bushes (allegedly) but (apparently) they are now safe.
So it's your choice. Just don't use a multigrade. It's too thin at gearbox temperatures.