The SM-318 is a truck transmission only application until around 1954 or 55, when it was also used for passenger cars. The SM 319 is thre same basic transmission, with a B-W overdrive fitted to the output shaft. The automobile trans from the late 1940's and early 50's is a much smaller, lighter-duty unit with a different bellhousing bolt pattern. Some of the 1940's transmissions also had vacuum-assisted shift linkage that gives a lot of trouble.

85W-140 is a synthetic gear lube which will cause incorrect operation of the 2-3 shift synchronizer, and will probably cause gear grinding. It would be much better to use straight 90 weight mineral oil in the transmission, and use the 85-140 in the rear end if you want to.
Jerry

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