Sometime in the previous century, I was involved with Allison transmissions; specifically, cramming them, the engine, the radiator, and various accessories into the back of City Buses, initial-filling them with fluid, and doing initial fire-up of the powertrain package. Then topping-off the fluid levels after the powertrain stabilized.
My head about exploded when one of our customers demanded we use 15W-40 engine oil in the transmission.
Turns out that Allison considered this perfectly acceptable. I'd have never guessed.
Kinda made me want to dump some TH400 lip-seals in engine oil to see if they would swell-up. Never followed through, though.
Another customer demanded a different kind of trans fluid other than the Dexron III-H that 95% of the buses got. This stuff was dyed red like trans fluid, but was thick like that 15W-40 engine oil. I don't remember what that fluid was called, or who made it. MAYBE this was the Allison "C-4" hydraulic fluid spec; which I guess is 10-weight. Most searches of C-4 trans fluid end up leading to Ford C4 transmissions. But a search of C-4 hydraulic fluid gets to the stuff I'm maybe thinking of.
