I've been pondering same thing for same reason (Gold GC has more Mg and less Ca than Green version), so searched and read about 8 threads about Magnesium. I found that Mg additives of the past were different than they are in newer oils. I think this quote by MoleKule sums it up best:
"There are also different Magnesium sulfinate and phenate formulations, and the latest formulations present no problems. Like ZDDP, it still a low-cost cost and tried-and-true compound for most oils."
Plus, I think the fact that some top notch oils use a sizable amount of magnesium compounds (NOT Mg atoms) speaks very highly of them.