Priobe,
Since you already have the pan off, check the gap between the rods. You gently push the rods apart and carefully use a fealer gauge. Do not stick it in and score the rod journal. Too much clearance here could cause your problem. It wouldn't hurt to check the end play in the crank. That should be checked before the rods are connected, but I think an excessive amount would still show up - not sure though.
Whoever said that the pressure relief valve does not control the
oil pressure at idle is correct, but that is kind of a microsoft type answer. Technically correct but useless in this case. In a normal functioning engine, at idle the pump is not pumping enough (volume of)
oil to generate enough back pressure to cause the relief valve to open, therefore it does not control
oil pressure at idle. However if the relief valve malfunctioned and does not close completely, it will cause low oil pressure at idle, as not all of the oil the pump is pumping is going out to the engine.
You should be able to watch the oil pressure climb with rpms until the pressure reachs the point where the bypass valve opens. From that point on the oil pressure should stay pretty much the same.
What is the oil pressure at 1000 rpm, 2000 rpm, 3000 rpm, 4000 rpm - hot and 1000 rpm & 2000 rpm cold?