There is a TSB about why they recommend Special Nissamatic D as best ATF .......low ash ATF. When some low to lesser quality ATF overheats it breaks down and deposits ash and varnish on clutch faces.
Synthetics are ashless [other than metallic additives] and the higher destruction temperatue assures they won't break down.
Dexron minimum spec ATF are made by dozens of companies without detailed specs you only know they are somewhere around the Dexron spec.
http://www.swri.edu/4org/d08/Abstracts/blueatf.pdf
http://gulfoilphilippines.com/...s.htm
Mixing ATF just assures that what ever the lowest quality component is that is the one that prevails.