The very first commercially-successful, mass-produced automatic transmission was the Hydromatic. At first, it used engine oil as "trans fluid", and it was intended to be changed about as often as engine oil.What's the H in Dex III-H?
Eventually GM created "Type A" "transmission" fluid, soon replaced with Type A, Suffix A.
The suffix letter "A" is important here. Major changes in the formulation got a number and suffix upgrade, minor upgrades got only a suffix letter upgrade.
The first Dexron fluid was "Dexron B".
Then Dexron II-C. Later, II-D, and then II-E.
Another big upgrade called Dexron III-F, (which is absolutely, positively NOT NOT NOT related to "Type F" fluid for Fords) then III-G, and finally III-H. Dexron III-H is the latest, bestest version of Dexron III. Well, except for Dexron III-K, which is newer, but only for manual transmissions.
The initial big-deal synthetic Dexron was Dexron VI-J. (As with other letter-based ID systems aside from Roman numerals, the letter "I" is not used due to confusion with the number "1") And, of course, just because GM is run by total morons, all the newer versions of trans fluid don't really follow this time-tested, multi-decade-proven system. Dexron HP, Dexron LV, Dexron ULV...hell, they're even using the Dexron name for non-automatic-trans-fluid lubricants.
Wikipedia "Dexron" has detailed info.
DEXRON - Wikipedia
en.wikipedia.org
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