I noticed this in the link provided by Xram:

Zinc is the amount of zinc used as an extreme pressure, anti-wear additive. The zinc is only used when there is actual metal to metal contact in the engine. Hopefully the oil will do its job and this will rarely occur, but if it does, the zinc compounds react with the metal to prevent scuffing and wear. A level of .11% is enough to protect an automobile engine for the extended oil drain interval, under normal use. Those of you with high revving, air cooled motorcycles or turbo charged cars or bikes might want to look at the oils with the higher zinc content. More doesn't give you better protection, it gives you longer protection if the rate of metal to metal contact is abnormally high. High zinc content can lead to deposit formation and plug fouling.

This makes me wonder, would a high-zinc oil help in an engine that was designed for leaded gasoline, but is forced to subsist on unleaded now? I know there are lead substitutes, but I don't know what they really are or how well they work.

1950 GMC 100
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In the Stovebolt Gallery
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