Steve is point on about the "making" of Dextron III no longer watched by GM. I recently changed out my tranny fluid and filters and used Delvac which is TES 295 approved. After the first change, you have to change again in 12 months or 15,000 miles. After the second change you can switch to the longer interval (both mileage and time).
Dave and Karen -- '02 Ultimate Advantage 36 C, 350 HP Cummins, Allison 3060 Tranny, 2013 Ford Edge, InvisiBrake