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So I'm reading the Service Counselor reprint (courtesy Max Merritt) which instructs service depts to use mineral oil in the transmission and overdrive.

Does anyone do that?

The present MIL spec for GL-5 gear oil requires non-reactivity with bronze. So modern gear oils are what is in most of those transmissions. The old Moss boxes specified motor oil due to bronze synchros the same way as Packard.

You can try a straight oil (available fro Lubriplate) and if the shift quality is poor change over to the GL-5 which is more grabby.

Joe, following the advice of others on this forum, I replaced the fluid in my transmission with 90W mineral oil (GL-1 rated) from Napa. This is available in 1 gallon jugs only and is pure mineral oil.

I could not believe the difference in shifting and driveability this made for my car. Granted, I have no idea what was in there before, from the previous owner, or how long it had been there, but I was very impressed with the difference it made.

Good ol' Internet. Transmission oil IS mineral oil, plus additives. Now that my leak problems are solved, I might change to straight mineral oil with next oil change visit. I'm sure I'll have to bring my own mineral oil, as I do oil filter.