I'm going to share my experience and my opinion on the three different ATF's.

The basis for this post is my having recently changed my fluid due to slippage. It was the second time I have changed the fluid in my '03 LS and it came at 197k miles. The first fluid change came at 100k for the same reason.
Backing up to the first fluid change, I dropped the pan, changed the filter, added 6 1/2 qts. of Dexron III fluid and 1 qt. of Trans X conditionong fluid. The fluid and filter change was a success and the car went on to give me another 95k+ miles of trouble free transmission service. Then somewhere around 195k miles, it began to slip and hold shifts again. We continued to drive it for a while but later parked it for the next two years because of the transmission problem as well as others, like a plugged radiator, bad steering pump and four bad tires.

Well, I recently decided to bring it out of retirement and make the necessary repairs to put it back in service. When it came time for the transmission service, I came back to this wonderful forum to reread and brush up on the ins and outs of the filter-fluid change. This time I read something that I didn't remember reading the first time, and that was to "Upgrade" to Dexron VI fluid even though the owners manual specifies using Dexron III. Having read of this "Upgrade" from more than one person (maybe even several), I decided to use it along with the Trans X conditioning fluid that I have had success with three times prior. This time would prove to NOT be successful. At least not completely. This time it did improve the slipping and it did get the transmission to shifting but there was still some slippage on take off and hard acceleration gear changes.

It was then that I decided to start investigating the properties of each of the fluids mentioned in the title. I found that each one has a different viscosity rating and that of the three, the Dexron VI has the lowest. Combine that with the quart of conditioning fluid I used and I had a fluid in there, with a viscosity rating of about 26. The Dexron VI fluid's viscosity is 30.2, by it's self. The Dexron III's viscosity rating is 32 and the High mileage ATF's viscosity rating is 36.6.

After doing my homework, I concluded that my fluid was too thin and therefore my line pressure was too low to allow my transmission to operate properly. With 500 miles on the fresh fluid change and my transmission still slipping, I decided to do another fluid change to the high mileage fluid to see if it would increase my line pressure and get the transmission to operating properly. Today I did that fluid change and I was right. The transmission performs perfectly with zero slippage or issues. The gear changes are quick and crisp now.

That is my experience and now for my opinion;
If your owners manual recommends Dexron III, use it. Dexron VI is not an upgrade. It is simply using the wrong fluid for your vehicle. And, if your transmission has more than 75,000 miles on it, you can benefit from using an ATF that is specifically designed for higher mileage vehicles.

As with engines, each transmission is designed to operate properly by using the correct weight of fluid and switching to a lighter fluid, because it is newer, doesn't mean you're getting a better product.

Dexron VI is not compatible with Dexron III and Dexron III is not compatible with Dexron VI. However, Dexron III and high mileage ATF are compatible.

I hope this helps others to have a little bit better understanding of the different ATF's available for our cars.