Dear John, Walt, and mid-60s Buick fans,
Hope not to stir up any controversy, but the upshot of all those readings I posted is that your engine is better off with an oil specifically formulated for period when your engine was designed. If the ZDDP additive isn't matched to the other components in the oil, it won't work as well. In the beginning of the "ZDDP" crisis, no manufacturer was formulating oils specifically for cars of the 1960s. That market is now large enough that you can buy an oil designed for our classic engines.
I was using Joe Gibbs oil because that is what the engine builder recommended. However, this market is now expanding rapidly so there are a number of vendors creating oils which claim to be intended for our cars. I would be careful to confirm that these products do indeed match the oils created during the 1960s, but if you find a brand that you are satisfied with, it is probably the way to go. After all, why hassle with an oil additive if you can get what your engine really needs included in the oil itself.
Cheers, Edouard