The best way to get to the bottom of this subject is to go right to the heart of the engine's lubrication system and start with the oil pump. The oil pump's job is to suck oil in from the crankcase and push it through the filter and oil galleries to the crankshaft and camshaft bearings. The steady flow of oil supports, lubricates and cools the bearings. As it moves up through the rest of the engine, it also lubricates the camshaft and valvetrain components, and fills the hydraulic lifters or cam followers to eliminate valve lash. Oil splash from the crankshaft also cools and lubricates the pistons and cylinders.

The oil pump by itself doesn't create oil pressure. It only pushes oil from one place to another. It's a positive displacement pump that moves oil as it turns. So as long as the oil encounters no resistance, the pump creates no pressure. But oil does encounter resistance as it flows through the filter, narrow oil galleries and bearings. Oil is an incompressible liquid, so as resistance builds so does oil pressure.

Temperature is also a factor. The viscosity of oil changes with temperature. Hot oil is much thinner than cold oil. Consequently, oil pressure is higher when the oil is cold because it is thicker and harder to pump through the engine.

When the engine is running and the pump is working hard to push oil through the system, some means of controlling pressure must be used to prevent too muc...