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As for switching from gear oil to ATF in a stick, that was done to ease shifting, NOT for durability.



I agree that's part of it, but the thinner oil get's into more places, so it protects better too.

With 80's honda manual transmissions, it was very common to see 5th gear burned out. The Common cause was adding gear oil instead of using the recomended 10w30 motor oil. The gear oil was too thick to make it out to 5th gear. The teeth would disintigrate.

Back to the oil in the auto debate , I remember at auto school (all those years ago) them telling us the base ingredient in dexron was 10w30 motor oil. The rest of it was all detergent, pressure, and heat additives.

It can be debated as to whether engine oil has better lube properites, but how often does an auto trans fail from lack of lube anyway?
Virtually never.
It is always heat, and lack of pressure that kills autos.

Although I don't think using motor oil is a good idea, I doubt it did any damage. It's just a good reason to go through it now that you know.