The fact an engine develops any oil pressure at all indicates the bearings are getting more oil than they need. If oil was being supplied to the bearings at exactly the rate it leaks away, the pressure would be zero. When a bearing has excessive clearance due to wear, scoring, wrong size bearings, etc., the oil runs off faster than the pump can supply it, so pressure drops to nothing. With proper bearing clearance, and an adequate-volume pump, the bearings get lubed, and the excess oil trying to force itself into the oil passages causes the pressure to rise. If pressure goes above the relief valve setting, the excess is dumped back into the pan before it reaches the oil system, and pressure is controlled by the relief valve. Most oil pumps are capable of flowing at least twice the volume the engine needs for lubrication. As oil thins and clearances increase due to heat, the pressure drops due to quicker runoff.
Jerry
