I would use Dexron III which is not synthetic. Mercon sometimes labeled Type F has different additives in it.
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One thing to watch for is the odor of the ATF. I recently mixed up some Ed's Red with, IIRC, Havoline (maybe Valvoline?) ATF. That stuff stunk!!! Ended up in the fuel oil tank. The stench was unbearable.
Next trip to the auto store had my nose at the bottle caps. Got some strange looks, but at least the Ed's Red is usable...
45_Colt
Tramsmition fluid good rust protection and cleaning and synthstic oil will keep rust away and clean
For protection at the shop I was part owner of we used Du-Lite Quick-Seal on all metal that was just blued and Hot water then cod water rinsed. Good for 2-3 years or more leaves a waxlike so fingerprints just spread out with no effect to the metal. It's sold in 55 gallon drums.