Quoting Wikipedia
"Dexron
The original Dexron 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 Dexron fluid had to be reformulated.
[1]
[edit] Dexron-II
Dexron-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.
[2]
[edit] Dexron-IID
Corrosion inhibitors were added to Dexron-II to address the solder corrosion issue. The resultant fluid, released in 1975, was called Dexron-IID. However, the corrosion inhibitor made the new fluid
hygroscopic to a problematic degree, and so had to be reformulated. Although the hygroscopicity was not a major problem in automatic transmissions, for which the fluid was originally intended, Dexron ATF:s were also used in other hydraulic systems, which is where the hygroscopicity was a problem.
[2]
[edit] Dexron-IIE
Dexron-IIE was introduced in an effort to address the hygroscopicity problems with the previous IID fluid.[
vague]
[edit] Dexron-III
In 1993, GM released new Dexron-III fluid. It is generally backward-compatible with transmissions originally filled with earlier Dexron fluids or with Type-A/Suffix-A fluid.
[edit] 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] Fluids asserted by their manufacturers to meet Dexron-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.
[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 during use (from shearing of the ATF) is lower for Dexron VI, resulting in the same lowest allowed final viscosity for both Dexron III and VI (5.5 cSt).
[5] The intent of the lighter viscosity is to gain an incremental improvement 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 (thins less in use) base stocks (the base oils, to which additives are added to make ATF).
[4]"
Also, from the Castrol website
For questions about our
consumer products please call 1-800-462-0835.
For questions about our
heavy duty or commercial products please call
1-800-255-4417.
I have found that you can get good answers by calling the maker of these products