Zinc additives have been removed from all modern oils sold for use in new cars, which all have roller camshafts instead of flat-tappet camshafts, like our old Eagles. The zinc prevents camshaft lobe wear. Th reason they eliminated the additive is that zinc is very bad for catalytic converters and oxygen sensors, and since all reciprocating engines use/burn some amount of oil, and the zinc is not necessary for newer roller cam engines, the manufacturers have stopped putting it into oil spec'd for new vehicles(oils with the latest API letter designation, whatever that is now). Even oils specified for deisel engines have lower amounts of zinc nowadays. I run Shell Rotella 15W40 in my old flat-tappet cam engines, and add the Rislone zinc additive.
As a side note, stay away from any oil filters with orange paint on them(not naming any names, but you know who they are) for all AMC engines, especially the V8s. Their filters are inferior(fall apart internally and plug up quickly and don't bypass oil properly) to almost any other brand.