Although I just changed ATF with regular Dino Dextron III, at 50,000 miles I changed from Castrol GTX 10W-30 to same weighted Mobil 1....yes, at 50K. I really hesitated because of the high miles, but from talking to a lot of people, decided to do it anyway. I drive over 100 miles a day to and from work and got tired of changing my oil about every month....Long story short, I'll never go back to dino oil. I have much better zoom zoom with the Mobil 1 and though I haven't measured it yet (changed it a few days ago), I suspect my gas milage has improved too. A big thumbs up for Mobil 1!!

 

this guy here tested this synthetic oil using a dyno:


http://www.chrono.org/dyno.htm

he concluded:

The increase has been 3.8 HP and 1.5 lb-ft of torque for just changing the fluids. If your stock plug wires are in good shape and you're already running synthetic oil, then the gains in power may be strictly due to the better MTX lubrication and cooling. I'm guessing that the synthetic MTX oil had the biggest impact.
* The average gain over the power band between 3000-6800 rpm was roughly 2 HP and 2lb-ft.