The original Dextron transmission fluid was introduced in 1968 as an improvement over the previous "Type-A, Suffix-A" fluid. Over the years, the original Dextron was supplanted by Dextron-II, Dextron-IIE, Dextron-III, and Dextron-VI, which is the current fluid. Because there are still applications for which Dextron-VI is either not suitable or not necessary, there remains a market for fluids meeting older Dextron specifications.
Dextron
The original Dextron fluid, like its predecessor Type-A/Suffix-A, used sperm whale oil as a friction modifier. The U.S.. Endangered Species Act banned the import of sperm whale oil, so Dextron fluid had to be reformulated
Dextron-II
Dextron-II was introduced in 1972 with alternative friction modifiers such as Jojoba oil. However, it made problems with corrosion-prone solder in GM's transmission fluid coolers and so had to be reformulated.
Dextron-IID
Corrosion inhibitors were added to Dextron-II to address the solder corrosion issue. The resultant fluid, released in 1975, was called Dextron-IID. However, the corrosion inhibitor made the new fluid hygroscopic to a problematic degree, and so had to be reformulated. Although the hygroscopes was not a major problem in automatic transmissions, for which the fluid was originally intended, Dextron ATF:s were also used in other hydraulic systems, which is where the hygroscopes was a problem.
Dextron-IIE
Dextron-IIE was introduced in an effort to address the hygroscopes problems with the previous IID fluid.
Dextron-III
In 1993, GM released new Dextron-III fluid. It is generally backward-compatible with transmissions originally filled with earlier Dextron fluids or with Type-A/Suffix-A fluid.
Dextron-VI
The fluid specification for Dextron-VI was introduced in 2005,and was first used as the GM factory-fill automatic transmission fluid for model year 2006. All Dextron-III licenses expired permanently at the end of 2006, and GM now supports only Dextron-VI fluids for use in their automatic transmissions. Fluids asserted by their manufacturers to meet Dextron-III standards continue to be sold under abbreviated names such as Dex/Merc, but the licensing system no longer exists. These fluids are not regulated by GM. Dextron VI is of a slightly lower viscosity when new compared to the prior Dextron fluids (a maximum of 6.4 cSt at 100°C for Dextron VI and 7.5 cSt for Dextron III), but the allowed viscosity loss during use (from shearing of the ATF) is lower for Dextron VI, resulting in the same lowest allowed final viscosity for both Dextron III and VI (5.5 CST). The intent of the lighter viscosity is to gain an incremental improvement in fuel economy by lessening parasitic drag in the transmission. Since Dextron VI is not allowed to thin out (lower its viscosity) as much as Dextron III during use, it requires the use of higher-quality, more shear-stable (thins less in use) base stocks (the base oils, to which additives are added to make ATF).