"He strongly recommended sticking with Dexron III."

Dexron III is any fluid that meets the minimum requirements of Dexron III (GM), which includes all synthetic ATFs that I know of, and roughly half of the non-synthetic fluids.

If it's ATF, it meets Dexron III (or later) or it meets Ford's standard.

Other than being better suited to operation in a cold winter climate, and better thermal transmissivity (for towing or heavy-duty), I haven't had synthetic ATF cure nor cause any problems, it's just ATF in a car.

As far as Rotella 15W-40 in a gas Mercedes, everyone has their favorite flavor, it isn't any better than the non-synthetic premium Mobil/Valvoline/et al oils. It also isn't a recommended viscosity for your car.

__________________

Gone to the dark side

- Jeff