The viscosity isn't the issue. It's the additives and rating of GL-1, GL-4, or GL-5. The old SAE 90W was pure mineral oil with no EP (extreme pressure) additives - now called GL-1. Pretty much the same thing as non-detergent 40W motor oil. Tractor Supply sells it as "Ford 90W transmission oil."
85W-140 as well as modern EP 90W gear oil are usually rated GL-5 with the EP additives.
The only potential problems with using the modern EP oil is some older transmissions are two things: Plugging wet brake bands when they exist, and slight corrosion of yellow metals like brass and copper. Many GL-5 oils now adays are certified as "yellow metal safe" and it's not an issue. In regard to your Farmall M, I don't know if it has any yellow metal to worry about anyway. Certainly no wet brake bands. That's more of an issue in Case and Oliver Cletrac crawlers.
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Today's Featured Article - Earthmaster Project Progress Just a little update on my Earthmaster......it's back from the dead! I pulled the head, and soaked the stuck valves with mystery oil overnight, re-installed the head, and bingo, the compression returned. But alas, my carb foiled me again, it would fire a second then flood out. After numerous dead ends for a replacement carb, I went to work fixing mine.I soldered new floats on the float arm, they came from an old motorcycle carb, replaced the packing on the throttle shaft with o-rings, cut new ga
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