20W-50 is too viscous. 10W-30 is just right. The splash oiling system in an old 216 requires a "thin" oil. Much of the lubrication depends on the oil being broken up into a mist or just being forced through the rod bearind by the dippers. 10-30 is just perfect as it flows well while cold and has good "body" when hot.
Also your engine does not require a special oil for old cars.
I would be somewhat concerned if the oil pan has never been off as there is a good chance that either the oil pump screen, which is very fine, or and oil pipe for a rod trough could be plugged-up. Either of these items could cause a rod bearing to burn-out.