My transmission dip stick says DX 3, yet Auto Zone told me Dextron VI
Yes, the factory fill was Dexron III. Dexron VI is the new version and is backward compatible. Was the transmission shifting well before the fluid change? If not, your mechanic did something wrong, but using the older spec fluid is fine. So more than likely they under or overfilled the transmission or forgot a seal on the trans filter. BTW: in some cases, if the vehicle has a lot of miles and never had regular fluid changes, a fluid change could bring on shifting issues.
Last sentence in Tom's answer says it all. If the mechanic did a FULL tranny flush, chances of problems with either fluid is unfortunately high, and to no fault of the mechanic. Introducing all the new detergents will do it. GMT360 platform, it's best to only change the filter and replace lost fluid with Dex 6.
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