Just had my tranny fluid changed out over the last week.
It may depend upon what type of material that some of the parts are made from. My C6 was built using Kevlar bands, other, premium parts, a shift kit and Dexron was put in it. I complained of mushy shifts, but their adjustments didn't help much.
A couple of weeks ago the shop had one of their transmissions from a Camaro that was drag raced quite a bit come in. The owner of the car called the manufacturer of the converter to ask why he was having so much trouble with the transmission since it had to be rebuilt a couple of times. After describing the other parts used in the tranny (similar to mine), the first question that the manufacturer asked was, "What kind of fluid are you using in the tranny?" The Camaro owner was using Mobil One. The manufacturer said, "WRONG!" With the parts being used, you must use Type F fluid - NO
SYNTHETIC!" They replaced the parts that were damaged, then put Type F in his tranny. They said it made a world of difference.
Because of the structure of my tranny, they could only change out around 50% of the fluid, but they did this 3 times over a week, so I have about 90% Type F, now. I has made a noticeable difference in the firmness of the shifts.
I have a tranny fluid temp gauge. The Type F runs 10-20 degrees cooler than the Dexron, which makes sense because there is less slippage, and , hence, less heat generated.