All transmission fluid is not the same. Car manufacturers invest millions of dollars in research and development to determine what is best for the cars they make. It is best to take their advice. With that being said, usually the only difference you may notice by using different transmission fluids is in how your car shifts from one gear to the other. Transmission fluid is basically hydraulic fluid like that found in braking systems, forklifts, or any other hydraulic system. It is a very refined oil. The refining process ensures that there is absolutely no contaminants in the oil. Contaminants such as water could lead to corrosion or rust within the system. Contaminants such as air or gases cause the fluid to be compressable, which would give the system a sort of spongy performance. When dealing with transmission fluid, the key factor is the thickness or weight of the fluid. If it is thinner than what is recommended for you car, it is able to be pumped faster by your transmission pump and is able to rush through the valve body of your transmission much faster, which causes the clutch plates in the tranny to be slammed together much faster than a thicker fluid would, thus causing your car to shift harder. When trying to get the most out of your drive-train, smoother is better, which is why I say, unless you are trying to tweak the way your car shifts gears for dragstrip or road-race purposes, stick to the manufacturers recommendations. With that being said, the difference between Dextron II and Dextron III is probably so minimal that a difference in shifting would be hard to detect. Hope you enjoyed the education. Goodluck.