If the cam was already broken in (which it was with 5,000 miles on it), then it doesn't need "high zinc" oil, though it's not going to hurt running an oil with more than the API/ILSAC RC maximum. A Euro 0W-40/5W-40 would be more than sufficient in the phosphorous department.
You said the engine code is D9, so it's a 1979 block. But you said it was also rebuilt. Do you have any of the rebuild details? If it has stock heads, it would have pedestal (non adjustable) stamped steel rockers.
There are clearly some significant details missing here.