BITOG did a study on this a few years ago. They took a few quarts of unopened oil that were made in the 60's and did an in depth analysis of it. Turned out it was exactly as it was made nearly 50 years ago....all additives up to spec, viscosity unchanged etc.
As stated already, the key is to make sure that the API specs are up to your engine's requirements. I bet they are.
1998 Beaver Patriot Monticello - 330 HP Cat "Ruby"
TOAD: 2017 Ford SHO "Luna" 365 hp