Since I've used Pennzoil since 1977 in everything I own I went to their oil recommendation site to get their take on what oil a 1973 flat tappet 455 should run. Their recommendation was 10W-30 Pennzoil platinum full synthetic, the API rating is SN.
As has been said current oil technology is vastly different and the self sacrificing zinc additives are not the only oil additives that will protect flat tappet cams. The current Pennzoil oils are also much cleaner as they are derived from natural gas rather than crude oil. They resist high temp burn off better as well as keep sludge accumulation to a minimum.
Just because you're running old technology engines doesn't mean that you need old oil technology to protect it.
The offshore lifters using poor quality materials and no crown on the lifter face, as well as improperly ground cams is what caused the mass failures of flat tappet cams and lifters. Since that was the same time that ZDDP additives were lowered, oil was easy to blame. If the parts are made of inferior quality materials, and machined incorrectly, no amount of ZDDP will keep them from premature wear.
