First I’ll discuss the worst case, actually having low engine oil pressure:
Oil pressure warning lights typically turn on at about 8 psi, which is low pressure. The light coming on doesn’t necessarily mean there is no pressure. So, even if the light is accurately telling you that the pressure is low, you wouldn’t necessarily hear engine noises right away. It can take some time for the low pressure situation to cause enough damage to result in engine noises. Of course, by the time you hear noise, the damage has been done and an engine overhaul may be needed.
If the pressure is low, most of the time this first occurs at idling speed, eventually getting worse and turning the light on while driving down the road. This scenario is somewhat consistent with your description of no light on the highway, then the light at idle and at lower speeds. It’s also common to have higher oil pressure with a cold engine, before the oil has warmed and thinned.
The certain test for this is to use a mechanical pressure testing gauge, attached to the hole where the oil sending unit* goes. The mechanical gauge system you plan to install would essentially do the same thing, though its accuracy may not be as good as that of a testing gauge. A good manual should have specifications for oil pressure (for a warm engine) at idle and/or at engine cruising speed.
If the pressure is low, the suspects include the oil pump, the pump’s pickup tube, the pump’s pressure relief valve, camshaft bearing wear, and crankshaft bearing wear. On some designs the pressure relief valve is part of the pump, on others it is separate from the pump. If I were going to pursue a low pressure situation, I’d first replace the pressure relief valve if it were separate. Next on the list would be to replace the oil pump, including cleaning or replacing the pickup tube if needed (sometimes a new pump includes a pickup tube, sometimes not). After that, were getting into engine overhaul territory. The pump related repairs would be experimental surgery, so to speak. If doing them doesn’t solve the problem, they would have served the function of diagnosis rather than repair.
Now I’ll talk about the hoped-for case, an electrical problem in the light system:
It’s not unusual for a sending unit to get weak and turn the light on when the pressure is still OK. You mention replacing the sending unit, but not how recently. If the sending unit is just a year of two old, it’s unlikely it’s faulty, but not impossible. Replacing the sending unit is a fairly cheap way to find out. However, if you plan to install the mechanical gauge system anyway, there’s not much point to replacing the sending unit just to see.
The leakage near the sending unit would be irrelevant unless it’s a sizable leak. Small leaks, even to the point of dripping, don’t usually affect the pressure noticeably.
The remaining suspicion would be that the wire to the sending unit is being grounded somewhere somehow. This could be anywhere along its length, part of which is in wiring harnesses, some even inside the cab. The most likely places, however, are under the hood where the wire, or its harness, have rubbed against something that has worn its insulation (exposing a bit of bare metal wire) and provides a grounding point.
One test for this is to wiggle the wire everywhere you can find it, with the engine running and a helper watching the light. If the light goes on and off with the wiggling, that pretty well confirms a wiring problem, and it’s just a matter of locating the bad spot. If the light doesn’t come on doing this, nothing has been proven conclusively.
Another test is to disconnect the wire. If the light is on (engine running) but goes off when the wire is unplugged from the sending unit, that pretty well confirms that the sending unit is registering low pressure. Touch and remove the wire end from the sending unit terminal back and forth several times to verify that the light goes on and off with the wire connected and disconnected.
Another test would be to disconnect the wire and drive. Tape up the end of the wire so that it cannot short to ground. If the light comes on any time while the wire is disconnected, it has to be a wiring problem. If the light doesn’t come on, nothing has been proven.
In cases where the symptom is intermittent to the point of defying reaching a definite conclusion, the thing to do would be install a “T” or “Y” fitting on the engine block’s sending unit hole, then install both the sending unit and a mechanical gauge. At some point the light will come on, and the gauge can be read to see if there is low pressure at that moment. Doing this is a pain, but it’s a way to eliminate all doubt.
Hope you find a wiring problem!
*“Sending unit” is the term commonly used to indicate a device that sends a signal to a dash gauge or warning light. Sometimes “sender” or “switch” will be used for the same thing. “Sensor” is commonly used to indicate a device that sends a signal to the electronic control unit (ECU, “computer”) that manages ignition and fuel functions.