Dexron-II, IID and IIE

Dexron-II was introduced in 1972 with alternative friction modifiers such as Jojoba oil. However, it caused problems with corrosion-prone solder in GM's transmission fluid coolers;[2] accordingly, corrosion inhibitors were added to the product. The resultant fluid, released in 1975, was called Dexron-IID. However, the corrosion inhibitor made the new fluid hygroscopic, which while it was not a major problem in automatic transmissions, made Dexron IID unsuitable for other hydraulic systems in which it was commonly used.[2] A further reformulation to address excessive hygroscopicity was named Dexron-IIE (GM Spec GM6137M).
Dexron-III

In 1993, GM released new Dexron-III fluid (GM Spec GM6417M and later GMN10055). It is generally backward-compatible with transmissions using earlier Dexron fluids or Type-A/Suffix-A fluid.
Dexron-VI

The fluid specification for Dexron-VI was introduced in 2005, and was first used as the GM factory-fill automatic transmission fluid for model year 2006. All Dexron-III licenses expired permanently at the end of 2006, and GM now supports only Dexron-VI fluids for use in their automatic transmissions,[3] although fluids asserted by their manufacturers to meet Dexron-III standards continue to be sold under names such as Dex/Merc. These fluids are not regulated or endorsed by GM.[4]

Dexron VI is of a slightly lower viscosity when new compared to the prior Dexron fluids (a maximum of 6.4 cSt at 100°C for Dexron VI and 7.5 cSt for Dexron III), but the allowed viscosity loss from shearing of the ATF during use is lower for Dexron VI, resulting in the same lowest allowed final viscosity for both Dexron III and VI (5.5 cSt).[5] The lower viscosity is intended to gain improvements in fuel economy by lessening parasitic drag in the transmission. Since Dexron VI is not allowed to thin out (lower its viscosity) as much as Dexron III during use, it requires the use of higher-quality, more shear-stable (less prone to thinning while in use) base oils.[4] The current GM specification that defines the fluid is GMW16444, which superseded the original specification, GMN10060.