The oil in question here is for a particular type which manufactures (less small engines as in mowers, generators, to name a few) and have moved on to. OHC ( not pushrod, flat tappet) , Fuel injection, tighter tolerances, modern engine designs. Race engines of the era and type this was designed for, small engines using flat tappet (L head or push rod), need the zinc for numerous reasons, just as cars pre unleaded fuel needed lead to cushion the valve seats. Not until hardened valve seats were the norm , that unleaded fuel became the standard. Same with these types of oil.