So you're cooking along in 3rd (or 4th) gear and .. 65mph. Right? You don't drive your pig faster than 65mph unless your name is
@wngrog or you've got something that isn't a F, 2F or 3F in your pig, right? It's a Land Cruiser, not a Land Speed Racer.
Anyway 65mph, 3rd gear, 33" tires, 4.10:1 gears. Call it 2700 rpm.
You've done 65mph for an hour on I-25 because you live in Ft. Colins, but the company called a meeting in Denver. Everything has gotten good and warm. The oil is hot, and has lost some of it's viscosity. You didn't really notice this, since the gauge shows good oil pressure, likely because the oil pump is still pushing enough oil that you're running against the pressure regulator. This is especially true if you've decided to run a 'high viscosity' oil.
Anyway, you exit I-25 and stop after 300 yards, because there is a stoplight at the end of the on-ramp. The oil is still hot, so viscosity is low. The engine is still hot, so while the heat source (your foot in the throttle) is now minimal, there are a couple hundred pounds of cast iron, aluminum, oil and water that are all well over 200F. The oil is thin, and the oil pump isn't turning very fast (you're at idle).
Your FJ55 is from the 70s, (unless you have the rare bird 67-69 vintage). it uses the tech of the era, including the "gauges" which are really indicators, not calibrated pressure meters. No fancy MEMs-based sensors here, it's all analog, with very little in the way of conditioning or protection circuitry. They run on the same 12V that runs your headlights. The 12VDC is supplied to the gauge, while the ground is at the sensor. A current passes through a wire-wound coil mounted within the needle's pivot and produces a magnetic field that moves the needle across the calibrated scale of the gauge. How far across the scale the needle goes - what reading it yields - depends on how much current flows through the gauge. This in turn depends on the resistance of the gauge's return wire which is again, grounded to the engine block through the sensor.
This is why if you managed to touch the wire that goes to the oil pressure sender straight to ground, your gauge will no longer work. The tiny wires in the coil in the gauge were never built to handle the current you just allowed through the gauge, and you now have an open in the coil and the gauge no longer works.
The resistance of the sensor depends on the oil pressure. Oil enters the end of the sensor which is screwed into the engine block and pushes against a diaphragm . The diaphragm moves a wiper inside the sensor which runs up or down a blade of known resistance this blade is connected to the return wire from the gauge. The more the diaphragm moves under pressure, the further down the resistance blade the wiper moves. So the resistance of the sensor varies with oil pressure and the needle of the gauge moves accordingly.
If you have an indicator ("idiot") light, then the circuit largely works the same way, except the 'sender' is really a switch. With enough (oil) pressure, it stays open, and the light is not lit.
You've been cruising (cruising, right?) down I-25, with the alternator happily making as many amps as everything in the truck needs, but now you're idling, so there isn't quite as much current available. Voltage drops some, and resistance is up because everything is hot, and that's just how things are. Hot wires and devices have more resistance. This is also true of the oil pressure sender. These things also conspire to show a slightly lower reading on the indicator/gauge.
You have a few 100,000 miles on the engine, so the journal clearances are not like they were when new. This permits a lot more of the oil to pass right back out the main and rod journals. Fortunately, it doesn't take much oil pressure to support the journals at low rpm. 10psi will do it up to about 1,000 rpm. But 10 PSI looks *really low* on a gauge without numbers. I can't find any metrology but I'd bet the FJ40/FJ55 oil pressure indicator shows around 7.5 PSI at the 'L' mark.
Oh, and that 200,000 plus miles have also put a bit of wear on the oil pump, so it's not quite as efficient at idle as it was when new. IDK if replacement vanes are available these days, but one used to be able to turn the non-driven gear over in a straight-6 chevy if you couldn't afford to replace the pump or gears. Check around page 120-121 of the FSM posted earlier.
@Cgn1976 will probably want to look the oil pump (and pressure regulator) over while it's all apart.
Back to the oil pressure regulator, which serves as a type of "safety valve" preventing the high pressure oil discharged from the oil pump to rise above a specified value by means of the adjusting valve. When the pressure is "too high" you can wash the bearings, leading to premature wear. Like any other mechanical part, it can wear, and if it fails to properly close, then more oil gets dumped back in the pan, and less is available for the main bearings, and subsequently rod bearings, camshaft bearings, and the rest of the engine.
This is especially true of the 'F' engine, and its oil pressure regulator. There is a pretty good writeup here:
Oil Pressure Regulator - http://www.globalsoftware-inc.com/coolerman/fj40/30C.htm
@Cgn1976 probably wants to go through the oil pressure regulator while he has the engine disassembled.