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
J S Auto Supply in Jamestown, N.Y.