I went to the local engine builder who does all the racing engines for around here and I was buying a couple cans of some specialty stuff w/ZDDP and got into conversation with them. They were of the opinion while it might be good for break-in, once a motor is ran in, it's not necessary. I was buying some to run in my flat tappet 327 and a 390 car I have (both well broken in) and he talked me out of giving him money.
Also talked to him about hardened valve seats and he was of opinion unless I was using as a daily driver, it would take decades before I'd encounter any harm running unleaded on old stock valves. I was surprised he had no interest in helping relieve me of some of my cash.
