The M transmission contains only straight cut spur gears and single straight cut bevel set (differential drive gears). Consequently a GL1 rated lubricant is entirely adequate.
GL2 is OK too - it's essentially GL1 with some added rust/oxidation inhibitor.
GL3 and higher rated oils contain EP (extreme pressure) additives. These are intended for highly-loaded hypoid and spiral bevel gearing. These additives are unnecessary and may be actually harmful to the "stone age" transmission in an old tractor. (Some EP additives cause INCREASED friction and wear in low tooth pressure-loaded spur gearing such as is in your M)
Bottom line: Buy the cheap GL1. It's a far better lubricant than the best that was available back when your M gearing was designed!!
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Today's Featured Article - The Cletrac General GG and the BF Avery A - A Bit of History - by Mike Ballash. This article is a summary of what I have gathered up from various sources on the Gletrac General GG and the B. F. Avery model A tractors. I am quite sure that most of it is accurate. The General GG was made by the Cleveland Tractor Company (Cletrac) of Cleveland, Ohio. Originally the company was called the Cleveland Motor Plow Company which began in 1912, then the Cleveland Tractor Company (1917) and finally Cletrac.
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