[roddy]

The number 1 killer of automatic transmissions (and fluid) is heat, not power. The best thing you can do to extend the life of an Automatic is keep the fluid temperature down. My car is a manual, so I can't speak on the temperatures that the auto transmissions run at, but I suspect they run on the warm side. If the aftermarket synthetics can withstand a higher temperature without starting to break down, then that alone makes them a better fluid and worth switching to. If my car was an auto though, the first thing I would do in the interest of longevity is add an auxiliary cooler.