Dexron 6 compared to Dexron 2 and 3

Dex VI is capped in the mid 6 range, but the Petro-Canada Dex VI product is 6.0cst @ 100C. Most Dex III fluids are 7.5.

While the fluid may be OK for the transmission in terms of protection, GM may have changed some programming of the transmission software to accommodate the Dex VI fluid. For instance, since Dex VI flows much better (and is much thinner) at low temps compared to Dex III, the transmission may behave strangely if operated under extremely cold conditions.

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The DEXRON-VI fluid is thinner when it's new (6.0cSt at 100C as you said) however the DEXRON-III fluids are typically not shear stable so the viscosity drops rapidly from about 7.5 cSt at 100C when new to about 5.0 cst in just a few thousand miles. In the end they can go as low as 4.0 cst.
The viscosity difference is the most visible feature of DEXRON-VI vs DEXRON-III but it isn't the biggest difference. The biggest differences are in thermal stability and friction durability (compare the specs and you will see).
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