Mains do affect oil pressure. There is only one replaceable cam bearing. Rod bearings are oiled by a squirt system.  The pump is a vane pump. It is the same system they used on the 88 tractors from 1946 on and they ran with about 12-15 psi. In about 1960 they increased it to about 25 psi. Have you checked your spring on the pressure regulator which is an external check? You have a gas engine correct? The crankshaft is not drilled like an automotive crank. Where are you located and I will try to send you some information.