Compression test/leak Down

[85Ram50]

OK today I tested the oil pressure. It started up cold at 66psi. I let it warm up and i came back to check and it was 18 psi. What is the issue??? AFAIK it is supposed to be north of 48psi when warmed up. It goes up and down when I rev the motor just as quickly as I rev it. Then sits at 18psi at idle.

Note: I did not know about the balancer belt which apparently runs the oil pump. It broke about a year after I bought it on an incline with a load I heard a big pop. It kept running so I didn't do anything about it. Never got an oil pressure light. When the head blew 11 years later, 5 years ago that is when I found out about that belt and honestly its only yesterday as I read up before doing the test this morning that I found out it drove the oil pump.