Actually zinc oil additives for synthetic is actually ZDTP, Zinc dialkyldithiophosphate, whereas conventional oil Zinc is ZDDP, Zinc Dialkylphophate.
During engine break-in especially in older engines, you should be using a formulated break in oil. There is intended wear necessary to the rings and cylinder walls to ensure proper ring seating. That is why the manufacturers recommend changing that oil after a brief period. With older engines the piston to wall clearances and bearing clearances are much "looser" so to speak, while modern engines have much tighter clearances and smoother cylinder walls at assembly, roller cams and lifters and well atomized fuel injection to ensure that there is less wash on the cylinder walls.
Zinc and molybdenum are often combined. The zinc has a polarity to it which effectively causes it to suspend itself and become a sacrificial element during the high-pressure / heat cycles of valve trains as well as TDC and BTC stokes of the rings and cylinder walls, the zinc is converted to an ash after the high heat event. The modern lubricants use less ZDDP as it tends to damage the catalytic emissions system. The molybdenum compounds provide lubrication in a more traditional sense.
In most discussions regarding ZDDP additives it is related to flat tappet cams and their failure due to improper break in.
just sayin..
