Back when your engine was built, 5W20 was not on the recommended list. I don't think it was even being produced.
I would be getting the diesel rated oil for the shear protection as the 85 has non-roller cam followers, which need the ZDDP{I think} protection. Given the non-roller and the shear, I would avoid using oils that depended on viscosity index improvers for their viscosity. A good 10W or 20W base oil, with improvers to bring it to 30 or 40 hot would be my best suggestion.
Translated, the oil numbers are a measure of viscosity at cold and hot temperatures. The first, 20W for example, means viscosity {thickness} of a 20 weight oil COLD, the second number - 30 or 40 - indicates when hot, this particular oil will have the viscosity that a good quality 30 or 40 weight oil has when it gets hot. That magic is done by long chain polymers that expand when heated. They make the oil 'seem' thicker when hot, but are subject to getting cut shorter, or sheared by the cam followers. Start with a good base, and you'd always have at least that base viscosity when the improvers are worn out.
tom