Oil + additive viscosity

I have had this motor running since 2006. Before this I had the same setup (same items) on a 318ci block and cannot recall this pressure drop. That block ran 70 + psi with 5w40 oil hot. This 360 block behaves totally different. It maybe different clearances or just being a bigger crank makes a difference. Anyway the check valve in the thread fitting is 5/16" in diameter. This is only made worse by the actual valve which consists of a steel washer type flap with a light spring behind it. It resembles like a PCV. This flap has four notches in it which when the valve opens the oil passes around the flap via its small notches. The filters here do have drain back valves built into them. Does this mean I dont need the check valve? If so I will modify one of the fittings (the one not fitted) by removing the check valve assembly in it and open up the hole to at least 7/16" to see what happens. It cant hurt it.
Regards
Billy D...