I wrote the Shell Answer Man about what oil to use in older Tractors in my cold climate in Northern Indiana. Here is his reply. Again I'm no expert but this is what the man told me when he wrote back.
Rather than use a current day gasoline engine oil, I think you’d benefit from the extra wear protection (important for flat-tappet/cam) of Rotella T6 5W-40. It has higher levels of zinc and phosphorus comparable to levels in the oil when the engine was designed. Current day oils have reduced phosphorus to help protect the catalytic converter.