I suspect the question is because this engine has flat tappets. As the API classifications advanced, the amount of zinc and phosphorus was reduced in the interest of pollution reduction.
I would use 15W40 diesel engine oil for this engine, this oil has a higher amount of zinc and phosphorus to protect the lifters.
Why do so many people argue about viscosity? I use whatever is left over in my engines; 0W16, 5W20, 10W30.