2005 C-6 calls for 12 quarts of Dexron III (H) ATF fluid

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Rear end per manual uses 75W/90 synthetic gear fluid. 2 quarts if I remember right.

Is there any benefit to switching to REDLINE synthetic for the A-4 auto trans? It wouldn't do any harm such as causing leaks? Would the transmission run cooler or are there any benefits that will be noticeable using the synthetic fluid in the automatic transmission? I believe the Dexron III (H) is not a synthetic transmission fluid.

In manual it says you never need to change the fluids in auto trans or in rear end differential unless there is a leak. How often do you change the fluids in the rear end and automatic 4 spd. transmission? Which products do you use synthetic or non-synthetic? Have you notice any differences switching to synthetic fluids on the above 2 components on the 2005 A-4 C-6?