The oil pressure drops at idle as the temperature of the oil goes up. The warm oil is thinner and flows much easier.
The pump is a positive displacement pump. It delivers the same volume of oil per revolution at all speeds. When the engine is at idle, the volume drops. When the engine is at higher RPM, the pump makes more volume than can be used by the engine, so a portion is bypassed back to the pan through the relief valve. That's why the pressure levels off and holds steady through the operating speed range.
As the bearing clearance increases with wear, the volume of oil that is required to pressure the system increases. Eventually the oil pump can't keep up so the idle pressure becomes lower and lower with wear. Heavier weight oil will help get the pressure back up. Problem is, the heavier oil will be much harder to move through the oil galleys, especially the long galley going up to the rockers.
The 15w40 weight oil is a good all around viscosity for the older engines. It's thin enough to circulate when cold, and heavy enough to keep "some" pressure at idle. A good rule of thumb is 10 PSI oil pressure per 1000 RPM. So 15 PSI at idle is plenty sufficient.
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