Originally Posted By: 901Memphis
Originally Posted By: SoNic67
I would NOT use a Low Viscosity formulation in that transmission, especially when it's already hard shifting. Unless you want to put it out of it's misery quicker...
All the synthetic formulations are LV, exception being Mobil 1 and Amsoil (they have both variants). They create less operating pressure than the conventional fluids, and with the worn transmission pump, pressure it will be even lower.
That's why the shifts hard when the transmission heats up - the fluid becomes thinner. Probably the additives are depleted, so it's viscosity drops very low.
For that transmission the best you can get now is Castrol HM. The drain and refill needed to be done at every 30-40K miles. If the fluid comes out grey, do again a drain and refill after some 200 miles.
They hard shift because of worn clutches tripping the max adapt mode because shift times are too long. DexronVI won't affect this.
I would use the Supertech DexronVI which is the same as the name brand.
Not trying to come off rude or anything but wasn't your 4T65e rebuilt? If it was then sure, go ahead and use Dexron vi since everything was assembled back to spec. That's what I would recommend in your situation.
Now on an original tranny with some miles on it that is exhibiting problems I for the life of me cannot figure out why a thinner fluid would be recommended.
IME, and for MANY others (visit some dedicated GM FWD vehicle forums) Dexron vi can cause issues in older transmissions. Switch back to a dexron III equivalent or a high mileage atf and the issue is gone. VISCOSITY is key here. Some of these older units just flat out need higher viscosity.
It doesn't matter how shear resistant dexron VI is. If the starting viscosity is too low for the tranny to operate correctly then it is the wrong fluid. Period. Use a higher viscosity atf and change it out every 30,000 miles and viscosity shouldn't be an issue.
I feel like this is common sense.