I have always used 50W motor oil in all my customers and my own transmissions and have never had an issue.. From Muncie 4 speeds to Clarke 5 speeds etc. 50W oil is the same viscosity as 80/90 gear oil but is a little more prone to creep so you need good seals and gaskets. This does not apply to the newer vehicles as you should use vehicle specific fluids. Even in the old B series, 2 stick Macks we used 50W.
Barry
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