IMHO,
Guessing will not solve the issue.
If your that concerned, you might send a sample of the fluid in for testing to determine what it is.
But if I were you, I would go with the MFG recommendation of GM/AcDelco Dexron VI.
I would also do it in steps.
Do a spill and fill along with a new transmission filter. then the following week do another etc until you replace the total amount of fluid that the transmission holds.
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