How about full pressure lubrication? Did those cars specifying non-detergent oil have filters and full pressure lubrication or splash lubrication without a filter?
I would also want plenty of anti-wear additives. I have seen oil specifically marketed for classic cars that isn’t meant for modern cars due to the emissions control systems getting fouled by the additives. You can also buy zinc additive packages in a bottle/can to add to regular oils when used in classic cars.
Then there is diesel rated oil which might not be a bad option for you. I run this in all my outdoor equipment without emissions controls and it seems to do quite well. They seem to start easier and run smoother, especially the ones with hydraulic lifters.