Dexron VI is the current version of GM`s long-time standby, and is marketed as being backward-compatible with all previous versions. However, some members (namely Starless Alex) have reported recently that all is not perfect, as the simple "Drain & Refill" leaves around 40% of the old ATF within the system, and it takes probably 3 repititions of the D&R procedure before almost all of the fluid has been replaced.
Good luck, and report back to let us know how you made out.
The road goes ever onward....
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When faced with choosing between two evils, always go with the one you`ve never tried before....