High iron (not lead ) in mobil 1?

I admit when it comes to oil I don't know to much. But i've been searching the site alot, no really, ALOT and from what i can tell use of mobil 1 seems to yield higher iron results in the used oil. What is the source of the iron. What are acceptable lead #s? If the iron is higher in Mobil 1, why are so many people using it? My guess it is the best bang for the buck over other synthetics. Is it the best full synthetic for the price?