Just a thought -
Be careful running heavy oils (20W-50) with high volume and/or pressure
oil pumps and
oil coolers. The higher viscosity
oil will put more stress on the gears running the pump. Run the lowest viscosity possible that produces the pressure you desire. Flow rate should count as well as pressure.
Also, it could be that much of the oil pumped just exits via the pressure relief valve rather than flowing through lengths of hose, an oil cooler and a filter.
Bob