The only API EOLC additive I know are Castrol Magnatec and M1 0W40 FSI would ask them to show me where ZMax is API approved. When they can't show you that, I would ask them to drain and fill the crankcase with fresh oil, without the additive. When they balk, then I would ask them to show in the owner's manual where ZMax is an approved additive. When they can't show you any manufacturer's approval, ask if they are going to give you a lifetime warranty to cover internal engine failure.
After the fiasco with Slick-50 and other similar products, I am very skeptical of aftermarket motor oil additives that make claims to "soak into metal", or other similar claims.
Z-Max ? Its oil. I don't think a little oil in you oil will be a problem.
But I would show them the line in the O.M. that cautions against oil additives. And they will say,
"You won't take Carroll Shelby's advice?"