'oil pressure gauge

If your oil is full, it's most likely electrical. If it's not electrical, then it's most likely something like a crack in the pickup tube, sucking air if I had to guess.

Problem could be a bad oil pressure sensor or the oil pump is failing. A mechanical oil pressure gauge needs to be hooked up to verify actual oil pressure. Problem could also be a bad stepper motor in the instrument cluster that drives the gauge.

Did the mechanic drive the truck with a mechanical gauge to check the pressure ? The engine bearings or oil pump could be worn enough to allow pressure to drop at idle after it warms up.

Well the warning light is supposed to come on if any of the gauges display a dangerous condition, such as no oil pressure.
You may have a faulty sending unit for the gauge, or a wiring problem, or the engine may actually be loosing oil pressure when the gauge drops.
You could replace the sending unit as a test, or have someone check the oil pressure with a mechanical gauge.

Check pressure with an external pressure gauge. Proceed from the results of that test.

My 2000 Jeep GC laredo had the same problem. I changed the oil pressure sensor (I think that is what it is called) and it now reads really high on my gauge but I havent had any more problems and I dont get that annoying ding when it hits zero anymore when I let off the throttle. In case you dont know, the sensor is located on the passenger side of the engine back towards the fire wall. You just have to disconnect the wiring harness and use a large socket to unscrew the sensor. Once you locate the sensor it should only take 5 minutes to replace. Hope this helps.

check the oil viscosity,it might be too old(driven for toomany miles)..

if thats not ityou might need to get the oil preasure lubricating pump checked,and id suggest you do it fast or theres a lot of engine wear and tear.

sounds like low oil to me........or a clogged filter. however if that is fine then it's probably a sensor

when you press the brake pedal the brake master cylinder vacuum booster takes vacuum away from the engine and that slows the idle speed just enough to trigger the low pressure oil light. Pull the oil pressure sending unit out and hook up an oil pressure test gage, if the pressure is above 15 PSI hot engine idle in drive replace the oil pressure sending unit.

Any time the oil pressure is 0 you take a chance of locking up the motor. The only way to know for sure is to put a pressure gauge in the oil pressure sending unit port on the engine and see what the pressure really is. At that point you will either have the peace of mind in the knowledge that the sending unit is really bad and needs replacement or you can then contact a repair shop and tell them what your testing has revealed. I wouldn't drive or run your truck while the oil pressure is at zero.

Good luck