I've been noticing that my transmission shifts much more firm/rougher than my brother'* 2000 SLE. His has 62,000 miles, my Aurora has 39,000 miles. I thought it should be the other way around. anyway, I checked my fluid and it is quite a bit below the hatched area on the dipstick. It looks to be about 3/4 of a liter low. Will a fluid level just a little bit below the hatched area cause the rough shifting? I didn't think that little bit of fluid would make that much of a difference really. What kind of fluid should I add to it? Will anything Dexron III work? It'* a 4t65E BTW that'* on the V6 Aurora.

I wouldn't think that a liter low would make much of a difference.
Are you checking the xmission oil when the engine is HOT and in Park?
As long as the bottle says Dexron III, then you can use it.
It should NOT say Dexron III compatible but real Dexron III.

Low fluid can cause pressure loss and ultimatly trans oil breakdown due to burning.