The old things ran a long time on single weight oil. I still believe in breaking them in on a 20 weight non detergent, but it is hard to find. A single weight oil would be my second choice and it is still available. I always add ZDDP to any engine with flat tappets. Once they are broken in, Mobil one makes a high mileage oil.
https://mobiloil.com/en/motor-oils/mobil-1/mobil-1-high-mileageIt seems to work quite well in old engines and extends the drain intervals a lot. I am hesitant to recommend a Diesel oil because Diesel generates different contaminants than a gasoline engines. But that doesn't keep a lot of guys from using Rotella 15W40.
Like Jerry says, this is a very controversial subject. I think everyone will agree that you don't want a synthetic designed for modern cars.
