From what I have been able to determine is if the trans has some miles on it or hasnt been "Freshened up" in quite awhile the seals, clutches, etc are use to the properties of the mineral based oils. Changing over to Syn, which has different lube properties can cause shifting problems cuz the old parts have been use to the other stuff for so long. If the trans is relatively new or rebuilt, switching to a Syn fluid may be a good idea, especially if you go to the longer change intervals like 100K miles. When I changed to Syn on one of my old transmissions it never shifted right until I went back to mineral based fluid.
Like I said, I just like to flush the fluid out of mine every 1-2 years. I can get a feel for what the pan looks like and what kinda wear material is being collected. The pan holds about 3 quarts of fluid where the converter and valve body hold around 10-12 quarts. When you do the standard change, you are just diluting the old fluid with a couple quarts of new. 12 quarts of Chevron ATF is $12. I do a complete flush and know I got good fluid. Figure at $3-4/quart for Syn, that becomes an expensive change. But if you do it once or twice over the life of the vehicle, its probably a wash.
Doing a full flush is pretty easy. Just disconnect the return line at the radiator (upper line) and connect a hose to it and drop it into a 5 gal bucket. Start the truck and run for about 3-4 seconds. Shut off and add 3-4 quarts of fluid. Do this 3-4 times and youll be flushed. If you dont want to break the line you can use a syphon pump and stick the tube down the filler neck, suck 3-4 quarts out, add 3-4 quarts, start and run for 3-4 seconds and repeat until the new fluid appears.
Of course, do this AFTER you have the trans warmed up and have changed the fluid and filter in the pan /forums/images/graemlins/thumb.gif