For DexVI, the friction modifier, metal deactivator and antioxidant chemistry's are more stable over a longer time duration.
What this does not mean is the level of additives is higher.
What this does mean is that the shifting performance and overall performance of the fluid will last longer, assuming no internal mechanical or hydraulic problems develop in the transmission.
Improved Metal inhibitors further keep the oil from oxidizing and forming varnishes.
DexVI is also much improved in the area of shear stability.
For a mineral-based Dexron III, the fluid may start out at 7.5 cST and shear down to 4.5 cSt within 30k miles.
For DexronVI, the fluid starts out at 6.0 cSt and may shear down to say 5.7 cSt.
The reason for the improved shear stability is due to two pieces of chemistry:
1. Some formulators use improved and highly stable VII polymers;
2. others use the new HM polymer esters not considered as VII chemistry.
As far as detergency, approximately the same amount of detergent chemistry is available in DexronVI as in DexronIII.
One of the reasons some manufacturers describe their DexronVI's as synthetic blends is that the DexronVI base oils are a mix of GroupIII, IV, and V oils.
The newer friction modifiers are not more "slippery." They have about the same friction coefficients as previous fluids, but are simply more stable over the life of the fluid.
If u have dex III, And that is what is recommended by the manufacturer, then I would use it, replace filter and call it a day. I can't find dex III so I use dex VI or valvoline dex VI compatible
You won't find DexronIII anymore because the licesnse is expired. You will find people using the Dex/Merc label since they still have the recipe for the old DexronIII batches. In some cases you are just buying DexronIII that is labeled as Dex/Merc. Dexron and Mercon specifications are not hard to meet with todays easily obtainable base oils and PI packages.
That being said there is not much reason to buy a dex/merc unless you are trying to save a lot of money or doing a flush since the synthetic blend DexronVI is so inexpensive and far superior to Dex/merc.