Actually, he told you wrong. None of the Mobil 1 formulations use Zinc. I'm not sure any pure synthetics do. However, conventional base blends like Syntec and some of the Valvoline synthetics do have Zinc in some viscosity ranges.
I don't have a clue about Zinc and Magnesium reacting. Zinc has been disappearing from motor oil because it's rough on the emissions equipment, and that has made it tough for old car guys to find oil that will help their cast iron, flat tappet hardware survive.
Zinc is excellent at reducing high pressure wear when there's metal to metal contact, like with flat tappet cams and press-fit wrist pins at the piston.