HOW IT WORKS: The illustration to the right is what a typical automotive "Gearotor Type" oil pump looks like with the cover off. [a] is the outer housing usually made of cast iron or steel, [b] is the outer rotor gear (with 5 internal lobes) and [c] is the internal rotor (with 4 external lobes).

As the center internal rotor [c] is driven by the distributor shaft, it also turns the outer rotor [b] which pulls in oil from the oil pan pickup through in orifice in the pump housing at location [1]. As the rotor assembly rotates clockwise the oil is drawn into the cavity [2] and sealed from the pressure side by the lobes making contact [3] and pushed to the pressure outlet [5]. As the inner and outer rotor gears mesh closer together it squeezes the oil up through the pump housing [6]  mount and into the engine's oiling system.
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THE RELIEF VALVE: The relief valve prevents over pressurization of the engine's oiling system by allowing excessive oil pressure to push open the relief valve [d] and to bypass the outlet and enter back into the suction side of the pump  [e] to prevent the pump from cavitating. A pre-set pressurized coil spring in the housing  [f] determines at what pressure the valve is set to bypass excessive pressure. Typically most relief valves are set at between 55 and 65 psi.
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WHAT GOES ON: The red arrows [7] indicate the increasing forces (darker arrows) being placed upon the housing by the outer rotor under operation, by the oil being pressurized between the two gears as it moves through the pump. Resistance due to the frictional metal to metal contact created within the pump during normal operation is substantial as shown in this typical Ford oil pump shown in this illustration. The metal to metal contact is listed below in order of significance. 11.75 sq.in. between the outer rotor and the housing contact surfaces [7]. 8.00 sq.in. between the rotor assembly and the floor and cover contact surfaces [8]. 2.1 sq.in. between the rotor tooth to tooth contact surfaces [9]. And .2 sq.in. between the center rotor shaft and the housing bore. That's a total contact surface area of 22 square inches of metal to metal contact spinning half the engine speed, at all times. That's 20 pump rotations per second in high gear at 55 mph in overdrive! Imagine how much heat is created in high rpm racing conditions! Additionally, high volume pumps have even more resistance not only in the oil it is pushing, but due to the increased surface area of the taller gears and housing. And for the most part, even the most popular performance pumps on the market do not take advantage of any smoothly polished surfaces, use bearings or bushing anywhere in their pumps at all. There is a lot of potential frictional restriction to be eliminated.
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CLEARANCES: Due to the requirements of mass productions and the rough factory surfaces, extra clearances are built in to the pump to prevent galling and seizure if they result into a slightly too tight of a fit. It is those excessive clearances where oil seepage and pressure loss can occur when the oil leaks through those clearances, especially when the oil thins as it heats up and the metal expands. One o the more critical sealing areas where proper clearancing is paramount, is where the edges of the inner rotor meet the outer rotor and the gears "wipe" the oil as it sucks it up from the pan [10] and pushes it along to the outlet and into the engine [11]. These clearances can not be adjusted easily to selecting a pump with the closest initial clearances as you can will be your goal. The clearances at the top and bottom of the rotor assemblies can be adjusted to reduce oil bypass leakage between the rotor and the top cover plate and housing floor [12]. This headspace is critical to control the clearance and oil leakage which can rob your of pressure and volume.