I'm all for using a better product when one exists for the application that needs it, but in this case, I still run Chevron DM3 (Dexron/Mercon 3 equivalent) year round, and buy it by the gallon at the local mcparts store whenever I need more.

The license for the Dexron name to be used on the Dexron3 product has expired, but the standard still exists, and the fluid is still made, it just can't be marked as Dexron any more.
Marketing, pure and through.

The Dexron6 stuff is a different base stock that it's made from, and while some transmissions are fine with it, I've heard that at least a few applications are specifically not accepting it, due to the different properties.
It's also supposed to be designed with different friction/lubricant behavior than the older DM3 ATF, but I don't know the specifics there.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...