I would be real careful using Synthetic in old muscle cars with flat tappet cams. Flat tappet cams REQUIRE an oil with Zinc in it. You really need a racing oil with Zinc, for instance, VR1 Valvoline, or Amsoil Dominator racing oil, Joe Gibbs racing oil. They have taken all the Zinc out of oils because it plugs up Catalytic Converters. I learned the hard way. At least put an additive with Zinc in it.