I just want to relate my experience with the oils used in the M135Cdn. When I joined the service in 1978, we were pretty much changed on the engine oils, going from the single weight oils (30 in summer, 10 in winter) to 10W30. We ran all the transmissions on straight 10W all year round, although the manual called for 30 in the summer. We just didn't waste the oil and time doing a needless oil change. By about 1982, as a results of the American's success using 15W40, we were directed to use that in all the military pattern engines. It was a little heavy at -20s, but since most of our deuces didn't have heaters anyway, nobody was in a rush to use them at those temps. And when we did, a can of ether and slave cables were the order of the day. We never changed the transmission oil grade though...10 weight summer or winter, and it stayed that way until the last of them left around 1987.
Currently I work for a museum here in Shilo that has a M135 with Dexron in the tranny. Works fine, but it took a bit of work to get the transmission to quit leaking. The oil change was done before I got here or I would have left it with 10w.
Re the 10W oil, the military also used that in the M109 transmissions, and it is also the spec for a host of heavy equipment transmissions in the Department of National Defense. However, I did note, while working in Afghanistan a few years ago, that some of that heavy equipment (the Zedelmeyer loaders for instance) had been changed on refurbishment to use 15W40 in it's transmission as well as for hydraulic fluid.
