Is higher stress for a couple hours before an engine tear down is likely to occur anyway (NASCAR), any worse than a lower level of stress for, say 100K miles before a tear down is deemed acceptable (airhead)?
BMW, and others specified SG rated oil, which is at the 1500 ppm level. There are photos on the internet of pitted&worn lifters from airheads. Now, it is likely we can't get a notary public to guarantee that those engines perhaps didn't have other issues, were run low on oil, etc. but that is pretty much the case for anything we find on the internet. I personally can't argue with any supporting proof that 1500 is the minimum or 1200 is, or 1000, or even 800. But in general, I think that more is better than less in this case, given the metallurgy involved and mechanical design of the engine with the properties of the motor oil at the time it was made.
Many of the automotive gearheads with old style street rods and weekend club race car owners with flat tappet engines are heavily into using the ZDDP additive on their engines, too, so it isn't just an airhead/BMW thing. I'm going to be conservative and favor the side of many others who've collectively more data and info than I have on this topic, and try to keep ZDDP in my airheads' crankcases. I'd certainloy be willing to read and learn about the experiments of others in this area, but am not willing to experiment on my own vehicles.
