I'm still trying to wrap my head around the appropriate oil type for this transmission. Vehicle is a 1985 Ford F-250 4x4 w/ 4 speed manual. Unit is a Borg Warner T19. Owners manual suggests straight 50 weight motor oil, or GL-4 80W-90 gear lube. Straight 50 weight is proving impossible to find, and 80w-90 gear oil is almost always of the GL-5 type which I've heard/read is corrosive to the yellow metals used in the synchros. Based on my experience trying different oils, the transmission is sensitive to oil weight in cold weather. When I first bought the truck, I used Pennzoil Synchromesh, whose viscosity as I understand it, is on the thin side; probably closer to ATF than 80w-90 gear oil. This oil was GREAT in cold weather, but I was unsure of compatibility with this tranny, so I drained it and tried SAE 90 GL-4 manual transmission lube. The 90 weight is miserable in cold-- if it's below 15F in the morning, I can hear the bearings in the tranny, and the engine RPM of a cold idle (high idle during warmup) drops about 200 RPM just turning the tranny internals until the fluid begins to circulate / warm up sufficiently. I'm at a loss as to what I can use for good performance in cold weather. With the 90 weight lube, shifting in cold (below freezing, without a 10-15 minute warm-up) is like trying to drag a spoon through frozen molasses. With the Synchromesh, it was effortless at any temp. But when I drained the Synchromesh, I thought I saw a silver sheen to the drain oil, indicating that there's a lot of extra wear metal, leading me to believe it's not the proper lube for this application. Any input, suggestions, info, tips, experiences, etc. would be much appreciated!