When I was roadracing motorcycles, Royal Purple zero weight was the
oil the pros used. Hard to believe zero wt.
oil would protect an engine, but it did for me. I raced a highly built single cylinder bike that made serious power. When I switched to RP I could see the difference on the dyno. My switch came as a result of getting help from Mike Velasco (Freddie Spencer's crew chief). Of course, we dumped it at the end of every race, but that's part of the cost of racing.
I wouldn't hesitate to use
synthetic oil in an FE, but then I have a hydro roller cam and it's less of an issue. I ran
Amsoil in my 390 FE and it didn't leak a drop. Synthetics are less of an issue when cars like ours sit a lot. They just don't break down like dino oil does.
If you're racing, and you dump it after each race, it doesn't matter whether you use synthetics or not. But.....that zero wt. Royal Purple is good stuff. I don't think I'd use it in my 428 though.