As mentioned, the question is what year and model are you discussing. there are different requirements as time progresses. From the beginning and up to '26 the countershaft assy is physically disconnected and the tranny lube must have sufficient viscosity to provide motion to that countershaft. A literal description is 'Semi-fluid grease'. An early formula was 2 parts 600 Wt Steam Cylinder oil with 1 part medium grease. The nearest equivalent is 1500 Wt . After the tranny design changed to a more modern S.A.E. design that viscosity reduced to around 600 Wt. up to late '28. Later models into the 30s reduced to 90Wt. So it matters which tranny you are talking about.
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