DEX III is recommended.
The military did switch to DEX VI (DEX 6, there is no 4 or 5) for their fleets, but in the civilian world the DEX VI is only approved for automatic transmissions, and all other applications still use DEX III that's been repackaged and a different part number on it.
Even if they put DEX VI in it, you can top it off with the DEX III as the 6 is compatible with the 3.
Here is a link to the GM tech bulletin about it
04-07-30-037E GMC Technical Service Bulletin
www.tsbsearch.com
short version, there is a higher rate of failure in non-automatic transmission applications when using DEX VI instead of DEX III. Enough higher that GM put out a bulletin, but not enough higher for the military to stock multiple fluids when one will work good enough (a lot of things are more about supply chain logistic rather than actually using the best product for the job)
Most fluids labeled DEX/MERC will meet DEX III, but not meet DEX VI. Just read the label to be certain.