Just my 2 cents, the standard pump can generate pressures in the region of 60psi probably much higher, and as far as I know the pumps return pressure can equal the flow pressure.
In practice there is very little restriction in the return circuit so the return pressure never gets anywhere near what the pump is capable of.
The pumps probably operate almost in "constant volume" mode due to their gear type design, so a restriction just increases the pressure, without much drop off in volume pumped. That's a theory, I've little to back that up!
The flow pressure should reach the pressure return valve limit (60psi?) when the oil is cold, but once its hot the pressure may drop to worryingly low levels on tickover, and even when cruising quite fast the PRV does not operate, well that's what some have said on this forum.
Given the above I am wondering if the oil is either returning to the tank via a leak in the pipe inside the tank? Or maybe too much oil is going through the rocker feed? (Easily checked by blocking it off temporarily). Having said that, the return volume to the tank must be at least be equaling the flow volume as otherwise the tank would run dry.
Another possibility would be lack of oil supply to the pump, it is largely gravity fed, so even a small restriction in that part of the flow circuit eg a collapsed pipe, or blocked gauze filter, could cause a problem.