What the army thought of using SAE 140 vs GO 90

[RANGER]

Here are a few quotes from the US ARMY in 1943, they were taken from Fargo Technical Bulletin #46G

"Our expert has proof GO 90 is better in transfer cases than SAE 140, because GO-90 is less likely to leak. It's all on account of the heat which SAE 140 generates when the vehicle is in operation. The gears of the transfer case have to do more work on the SAE 140 oul (because it's a heavier oil), thus generating more heat in the oil. This lubricant usually warms up to a temperature of 235 to 260 degrees Fahrenheit, wheras GO 90 seldom gets above 190 to 210.
These higher temperatures have two results:
(1) the SAE 140 gets thinner as it gets hotter. By actual test, SAE 140 has the same viscosity at iys operating temperature as GO 90 has at its operating temperature. Therefore, one will leak just as fast as the other if leakage is going to occur.
(2) More serious, the overheated SAE 140 will harden the oil seals, increasing the wear on the companion flange hub, and thus actually cause leakage."

Remember that leather seals were more common to the WWII Army vehicles. The claims of the relationship of viscosity to heat is valid. I was once a supervisor of a laboratory that was responsible for testing the viscosity of millions of gallons of petroleum product for the government and can relate to it.