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 - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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