Using a synthetic would be fine.
Blue Oval's Ford from that era, specified 5,000 mile oil change intervals, and he is following the specification. I like that idea. Follow the OEM specification. Like Blue Oval, I follow the OEM spec OCI, no matter what kind of oil I have in the engine.
However, a lot of other people would send in oil samples for analysis, and try to "dial in" exactly how many miles they can run that oil of choice in their engine. Nothing wrong with that either. But if you simply pluck a number, like say 10,000 miles, from out of nowhere - I would advise against it. Nobody knows what any oil will do, in your engine. If you have an interest in extended oil change intervals, then you should have your oil analyzed. You would also have to do a lot of analysis to compile enough data, for you to accurately pinpoint just how long a certain formulation of oil will operate in your engine.