I tend to agree that a stock '56 cam would be protected fine with an SM 5W30 or 10W30. I also prefer a 5-10W30 in the sbc. But since this classic vehicle should be given maximum protection and is going to see oil changes only once a year, I think something like Rotella S 10W30 if it were available would be a good choice and so Rotella T Synthetic 5w40 would be probably the best choice considering.
If I were to use a 5W30 or 10W30 I'd change the oil every 6 months regardless of mileage. Actually that's probably wanted I'd do just use the dino at 2x/year OCI. Even if I ran synthetic I'd probably still change it twice a year since this is a carbureted, points ignition, flat tappet, non-pcv valve application.
If I were to use a 5W30 or 10W30 I'd change the oil every 6 months regardless of mileage. Actually that's probably wanted I'd do just use the dino at 2x/year OCI. Even if I ran synthetic I'd probably still change it twice a year since this is a carbureted, points ignition, flat tappet, non-pcv valve application.