For your "REO" you should use a 15W40 engine oil, non syntetic, API class CD or better (MIL L-2105-B or better). The problem is not so much the oil, the problem is based in the oil seals of the engines. The "old" engine seals are NOT designed for the syntetic oil, they will leak a lot. The seal problem is not only of the radial seals, also the paper or cork seals. The engine will be an oil well!.
And with regard to enviromental protection we should care for not beeing oil wells. The "old stuff" beside that you will never get completely dry, so there is no reason to make them more leaky.
Classically the Army requires a W30 oil for summer and a W10 oil for winter below 5 °C (or something like 40 °F). But these oils are usually not available any more. So 15W40 is the best compromise.
Wolf