Respectfully disagree here, Eric. With a georotor style pump, The close tolerance between the gears acts as a seal between the suction and discharge ports. If the pump wears excessively, then it can't maintain a seal to effectively pump and maintain pressure. This could either: 1. result in oil starvation to the remote parts of the engine, as the pressure drop across the system may be sufficient to prevent oil from reaching these areas or 2. even it it's pumping 'some', it could still indicate that other parts are excessively wearing/worn as well and it's time for a teardown and rebuild. But without knowing exactly what the pressure is, I won't be able to identify that, and the idiot light wouldn't trigger in time to identify this either. This could be especially important in a well running, but high-mileage engine. SO yes, if oil pressure is 40 when it should be 90 (cold start) or 10 when it should be 35 (warm cruise) it really DOES matter.
I have a mechanical oil pressure gauge on my '98 Tacoma with the 5VZ-Fe engine and about 190k miles. I know what pressure should be at warm idle and at highway speed so I can monitor these, and if I see either that is outside (below) spec I'll know it's time to start investigating this small problem (worn oil pump) before it turns into a big problem (spun bearing, broken camshaft). With the on/off switch and the 'false' pressure gauge on the 3.5EB, by the time pressure drops low enough to trigger a low pressure warning there may already be severe damage occurring to other internals of the engine, esp the turbos.
