I'm sure there are people that can provide more details, but if I may.....
As transmissions got newer and especially when computer controls started to be used, transmission fluids were designed to work specifically with the new designs, which vary in how hard or soft they shift, the holding/friction properties and any other factors that may apply. Viscosities also vary between transmission fluids. The wrong fluid fluid can affect how a trans shifts and can lead to shorter life, whether it be from shifting/grabbing too hard and breaking parts to slippage, heat and burning up the bands and clutches. Transmission fluids are a very complicated product. I hope there are some trans techs that can provide more info.
