I found this by doing a GOOGLE Search:
"Automatic transmission fluids. ... Over the years, the original Dexron was supplanted by Dexron-II, Dexron-IIE, Dexron-III, and Dexron-VI, which is the current fluid. GM has upgraded the Dexron specifications over the years; newer fluids are generally but not always backward compatible with previous fluids."
The brand Dexron includes not only include an automatic transmission fluid - perhaps it used to but not anymore. Read this:
"General Motors is expanding its DEXRON® driveline lubricant products beyond transmission fluid and gear oil to include a comprehensive family of fluids designed to protect and prolong the life of Chevrolet, Buick, GMC and Cadillac vehicles."
So, make sure you're getting the correct Dexron fluid for your usage.