Low oil pressure
 

I have a 1996 Gen 6 engine that was recently rebuilt. It always shows low oil pressure after about 30 min of running at about 3000 rpms. I mounted a second mech oil pressure guage in one of the plugged outlets on the remote oil filter. I took it out yesterday and after running 30 min. the electric guage that is hooked to the sending unit at the rear of the block goes down to 5 lbs on the guage, yet the mech oil guage that comes off the remote oil filter still showed 50 lbs. I put an additional mech gauge with a tee where the elec. sending unit branches off and it verifies the 5 lb of pressure there. There is no knocking or valve tapping in the engine and I ran it that way all day yesterday. At idle when the engine is first started cold both gauges show around 40 lbs. Is the remote filter an accurate area to be measuring the pressure? Is the pressure relief valves in the block doing something that is preventing oil from getting to the elec. sending units? I replaced both of the pressure relief valves during the rebuild.
Thanks for any feedback.