Synthetic oil use on flat tappet cam after break in

Plus, aside from the possible cam wear issue, unless the engine was specifically built for synthetic use, thin full synthetics don't play well with 60 year old specs for bearing tolerances and clearances which can play havoc with oil pressure, and gasket and seal seepage, among other things.
I figure they were designed around Dino oil, and just feel uncomfortable running synthetics in them. The additive packages in the current crop of fossil oils (with the possible exception of the elimination of ZDDP) is light years ahead of what it once was, so they're still a quantum leap forward...