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You know, I've been thinking about this too (ZDDP). I don't know if newer engines have better metal, or if they are designed with lower leverage pressures (there must be a mechanical engineering term for this). It's my understanding that without these additives, it's harder to form a consistent oil film under high pressures. Pushrod engines without roller rockers and early OHC engines are very demanding in this area.
Supposedly, there are additives that contain twice as much ZDDP at the same price as the GM additive, but with 9 quarts of oil to change every 3000 miles, this is going to get expensive if it's needed (+20 an oil change).
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