Man, it's the OIL PUMP, without which your motor turns into a very expensive and heavy paperweight or yard ornament. I like oem cast iron pumps. I once bought and aftermarket (melling, I think) and took it apart and measured the clearances between gears and wall and side of gears and housing. Both were outside factory wear limits. My old oem pump was still well within specs, no scores or wear marks, and it went back in and the melling went back to the seller.
Water pumps are not as serious, but many aftermarket pumps have crappy impellers (stamped bent metal) and have low pumping efficiency. I'd buy quality, not necessarily oem, especially since the 2.6 pump is faily easy to get to.
Replacing the water pump necessity is obvious. It leaks or makes horrid noises. The oil pump is a little different. If you have lowish oil pressure, it's probable the problem is with the crappy electric pressure sender for the gauge, and the crappy gauge itself. My diagnostic tree starts with changing to thicker oil, checking the sender and gauge (with a reliable mech gauge on a rubber hose, or switching to a mech cabin gauge (the best solution), and only then worrying about the oil pump or too large bearing clearances in the engine, which a new pump won't fix.
The electric senders are a diaphragm connected to a rheostat, so that pressure on the diaphragm pushes the rheostat into a lower resistance region of the coil, allowing more current to flow to the gauge, driving the needle higher. The diaphragms develop leaks (almost all of the older ones I've seen have evidence of a leak on the housing). The wiring harness for the gauge develops higher resistance over time, lowering the current to the gauge and causing a false low reading. Mech gauges cure all this. You can use a std sae 1/8"npt tap (most hardware stores, and pretty cheap) to hog out the hole that taps the oil gallery for the electric sender so you can use the 1/8npt fitting in most mech gauge sets without having to troll around for a BPST fitting adapter. You can also use a 1/8"npt tee and very short pipe nipple and plumb in a low oil pressure light (there's a place for one in the dash, already wired in, I think) and the mech gauge fitting. It's easy to miss low oil pressure for long enough to have serious engine damage, but the red light catches your eye at once, usually in time to save the motor.
