In a high distance engine, low oil pressure is usually due to a combination of worn main and rod bearings and crankshaft journals. The oil pump does not generate pressure. It makes flow and the resistance to that flow causes pressure. Resistance is the product of the orifices in the engine block through which the oil flows, and the amount of gap between the bearings and crankshaft journals. As the bearings wear out, gap increase allowing increased flow which reduces pressure.
Note here that it doesn’t take much of an increase in bearing clearances to cause a noticeable drop in oil pressure as well as noise. This applies to all cars from brand new engines to high mileage ones.
Excessive bearing gap or clearance ( usually more than about .001 inch per inch of diameter of the crankshaft journal) can create up to a 20 percent or greater drop in oil pressure, which may have an adverse effect on lubrication elsewhere in the engine ( the camshaft and upper valvetrain, usually in overhead cam engines). Excessive bearing gaps will also increase engine noise and pounding, which over time can lead to bearing stress and failure.
Recommended bearing clearances may vary a great deal depending on the engine application, however many engine producers today aim for about .001 to .002 inch clearance in the main and rod bearings.
Excessive gaps somewhere else in the engine can also reduce oil pressure. These are wear in the lifter bores, excessive gaps between the camshaft journals and cam bearings, and excessive end play in the cam. Usually , any cracks in the oil galleys, leaking galley plugs, or leakage between the oil pump and block will also affect the oil pressure.
One major solution for low oil pressure due to excessive bearing gap is to reduce the clearances by changing the bearings or overhauling the engine. Having some new oil pump or a higher pressure pump installed won’t help because the bearings have too much a leakage rate to hold the required pressure. Installing a higher volume oil pump can actually increase flow and regain a little lost pressure. But the underlying clearance problem will still be there, which will speedup bearing noise, wear and fatigue.
Another common cause of low oil pressure is wear or excessive gaps in the oil pump itself. Though specifications vary, as a rule gear type oil pumps should have less than about 0.003 inches of end play between the gears and the cover. The gaps between the teeth and pump unit should usually be less than about 0.005 inches. With rotor style pumps, the gap between the outer rotor and pump housing unit should usually be less than 0.012 inches, with no more than about 0.010 inches between the inner and outer rotor lobes. Excessive clearance inside the pump will limit the pump’s ability to pump oil efficiency, which reduces flow and pressure.
contd reading;http://www.automobileglobe.com/what-are-the-causes-of-engine-oil-shortages/