Two cents worth as a design engineer... Id repeat what Mike said.. under drive the PS pump, there is little assistance required at speed, use a
synthetic fluid (I had heard one P Brock used a Mobil fork lift trans fluid on a Monaro he raced in Targa) and add a cooler or even a simple cooling loop of exposed / finned alloy tube to increase the volume of fluid in the system all of which will support heat dissipation.