I'm no engineer or chemist, so I can't detail out the reasons. But it does have to do with application, clutch material, fluid pressures, valve body programming, etc. You can also throw in Type F and Mercon transmission fluids also. I try to stick with manufacturers recommendations unless there is ENOUGH proof that something else will work as good or better. One example is using conventional or synthetic Type F fluids in race torqueflites.

But from what I understand, the ATF 4 is a higher quality fluid than Dextron III and may be synthetic.