Last trans service I did, I did a little research. At work I can get Dextron/Mircron III for dirt cheep. ATF III is almost $30 a case around here. What I found is this: The Dextron is a "grabbier" fluid. The ATF III is a more slippery fluid. The Dextron can cause the clutches to chatter instead of (as it's designed) slip slightly into place. They make an Additive (I can't remember the name now) that the parts stores have that can alter the chemical composition of Dextron to work in our vehilces. If I remember right it took me one bottle of it at about $10. I'm SURE I saved at least that in trans fluid.
Josh