DexronVI is just a more shear stable version of DexronIII. It starts out at a lower KV100 but doesn't shear as much as the old mineral DexronIII would.
I would use up what you have on hand and then after a short interval, upgrade to a DexronVI or synthetic multi dex/merc like Maxlife, Amsoil, Redline.
http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3014880/Improvements_in_DexronVI_over_
Originally Posted By: MolaKule
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.