Hi volume oil pump

Before you tear the motor aprt, have you checked the oil pressure sender beside the distributor? On one of my cars that was the reason for this. Next, is there any leaks on the oilfilter adapter? That might also reduce the pressure.

Then there is the pump. I think you can co with a high volume. They work just fine. It's the heart of the engine It's like put in a peacemaker.

What oil do you use? I usually use 10/30. That works just fine for me

Good luck