M1 5W-30 EP HM is simply a better match for the Windsor architecture than ESP. It responds better to higher phosphorus anti-wear, more robust detergency, higher ZDDP, and additive chemistry aimed at older metallurgy and seals. EP HM provides all of that, ESP by design, does not.
ESP is a great oil for the engines it was designed for. ESP's chemistry is optimized for emissions hardware and modern Euro engine design. EP HM's chemistry is optimized for older engines with looser clearances, higher wear demands, and no emissions constraints.
I just think the use case of an old school 5.0L Windsor is better served by an oil more geared to it's requirements rather than approvals for engines that are nothing like it