General Motors (GM) produced the Dexron line of automatic transmission fluids beginning in the 1940s. GM improved upon the original lubricant and upgraded through multiple stages including Dexron II and Dexron III. Dexron III (H) and Dexron III/Saturn are upgrades to Dexron III fluids, and Dexron VI became the official replacement to Dexron III.
Dexron VI fully replaces Dexron III and prior evolutions of the Dexron fluids. GM manufactured the liquid for compatibility with transmissions requiring earlier versions of the fluid. Dexron VI works in any vehicle that requires ATF-A (the original automatic transmission fluid) or a Dexron variation. GM's newest vehicles require the use of Dexron VI in the transmissions, however.
Dexron variations prior to Dexron VI are not forward compatible. GM does not support the use of an older version of the Dexron fluid in any vehicle that calls for Dexron VI. Moreover, you cannot use an ATF, Mercron or older variations of Dexron in vehicles calling for Dexron III. Dexron III (H) is fully compatible with Dexron III, and GM engineered Dexron III/Saturn specifically for the Saturn line-up of vehicles.
GM terminated all licensing for the production of Dexron III in 2006. The company advised GM vehicle owners that any other manufacturer's claims made of compatibility past 2006 without official GM endorsement would be inaccurate. GM informed the public that the only fluid compatible with Dexron III was the replacement Dexron VI. The automaker then authorised licensing of Dexron VI; GM publishes a list of companies and product names that comply with its specifications for the fluid (see link in Resources section).