Dexron-III (3) was phased out in favor of Dexron-VI (6), and Dex-VI is backward-compatible with previous Dexron fluids. While many multi-vehicle ATF’s claim compatibility with older Dexron fluids as well as Mercon-V (which is a Ford specification), I would not recommend Mercon-V in your GM application. Stick with Dexron-VI if you prefer a synthetic fluid, or use the Dexron-III formulations from a reputable supplier.
Assistant Manager
J S Auto Supply in Jamestown, N.Y.