Had a Mobile Mechanic swap a supposedly freshly fully rebuilt 3VZE into my 90 Truck 4x4/ Automatic. The oil pressure is good when cold and moves with throttle. But soon after it warms up it goes down to nothing and does not move with throttle. Replacing the Sending Unit did not help. Checked the Pressure with a mechanical gauge and it read the same. The crazy thing is there is no noise. What all could cause the oil pressure to do this? I am not a mechanic. Oil Pressure Relief Valve? Oil Pump? or was it a bad rebuild? I am using 3.8 quarts Castrol GTX 10/30 and 1 Quart Lucas Oil.

edit: sb5 mentioned a bad seal inside the oil cooler possibly. I'll check.