This is from the Joe Gibbs break in oil site section and is a good description of the function of Zinc additives:
ALL "ZINC" IS NOT THE SAME
The oil additive Zinc DialkylDithioPhosphate (ZDDP) works because it is a polar molecule, so it is attracted to ferrous metal surfaces. However, Zinc (ZDDP) is not a lubricant until heat and load are applied. ZDDP must react with heat and load to create the sacrificial film that allows ZDDP to protect flat-tappet camshafts and other highly loaded engine parts. The Society of Automotive Engineers’ Automotive Lubricants Reference Book states,“ZDDP is the predominant anti-wear additive used in crankcase oils, although it is a class of additive rather than one particular chemical..” Not all ZDDP additives are the same. Some zinc additives have slower “burn” rates that require more heat and more load to activate than other zinc additives. As a result, not all “High Zinc” oils have the same activation rate. The Joe Gibbs Driven BR Break-In oil uses a “Fast Burn” ZDDP that activates quickly.
THE CRITICAL BALANCE OF DETERGENTS TO ZINC
Detergents and dispersants in the oil complicate the situation. Detergent and dispersant additives “compete” against zinc in the engine because they are polar molecules as well. Detergents and dispersants clean the engine, but they don’t distinguish between sludge, varnish and zinc – they clean all three away. Modern API certified oils contain higher levels of detergents and dispersants due to the exhaust gas recirculation (EGR) systems on passenger cars and diesel trucks. The “old school” theory on engine break-in was to run non-detergent oils, and this allowed for greater activation of the zinc additive in the oil.
There seem to be 3 types of Zinc compounds in use, ZDDP, ZnDTP and ZDP. I really need to know which one is in the Castrol oil to figure out if it has enough total Zinc or not, BUT there is no way of knowing that and all I will get is a simple total figure from the UOA.
The oil lab geeks insist it is a better diesel oil than the Synthoil I am using at present and that the lower Zinc figure is not going to matter, but only time will tell. The various studies I found in Google seem to think a C3 oil is better than any older spec, which is mostly why I am going to give it try, for one OCI.