I was given Dextron 3 (III ?) by Gene Berck for my 1954 Hydra-Matic.
Original fluid was Dexron or Type-A/Suffix-A
Wikipedia: Dexron-III
In 1993, GM released new Dexron-III fluid (GM Spec GM6417M and later GMN10055). It is generally backward-compatible with transmissions using earlier Dexron fluids or Type-A/Suffix-A fluid.
I filled the empty H-M with about 5 quarts (to the fill level on the dip stick)
Then, start the engine (rear-end up on jacks).
You will hear slurping sounds, and may then slowly add up to required fill (9 qts ?).
8.5 qts on refill
9 qts on rebuild
If no one offers a better answer, glue a magnet to the fill plug?
