https://www.sae.org/publications/technical-papers/content/952344/ You may want to research this topic before you follow the "traditional" wisdom, aka "wisdumb". You're not likely to be sustained driving much over 2-3000 rpm, so what's the fuss? Making money for the sellers.
I had another SAE research level report but can't seem to locate at the moment, but the upshot is the same. After a certain amount, the excess ZDDP offers no additional wear life! ie, not proportional to content! In this article summary, 2x same as 1x at 100k miles.
What I recall was 10-15% goes out during a change cycle mileage, so anything more is just ending up in the used motor oil recycle drum. Waste/pollution - that's just one reason they decreased the level.
It's not going to "grow' new metal on old worn out parts either nor make them last longer. Once they get worn, the friction will be worse, regardless. These cars weren't designed to run for hundreds of thousands of miles - technology didn't exist back then.
Another case of "old" technology being put forward as "better" or "correct" in a future technology state/time. Many examples in the old car hobby. Humans are mythologically driven when they don't understand. A guild/craft based hobby.
disclaimer;I don't recommend nor disparage products nor uses of said. Proceed at your own risk.
ps: A quick search shows your oil appears to be still available, just maybe not where you usually buy it.
The modern "racing" labelled oils tend to be higher zinc, as the problems of catalytic convertors and crankcase emissions do not apply to the race track - at least not at the moment. The rpms are also much higher 2-4X than the antique autos.
Some people have resorted to diesel oil in gasoline engines and that's not recommended. No harm as I understand, but costly. (I own a diesel)
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