2003 Sierra Pickup 4.8 V8, oil pressure reading very low. I need to trouble shoot. Where is the sending unitl for the oil pressure gauge?

Probably oil pump

Your gauge shows a high reading

You have to determine the actual oil pressure
with a gauge,then trouble shoot the dash gauge,
for whatever it's problem is

Your talking about low oil pressure, not level, correct? Hope you can read diagram. The top wire, oil pressure sensor should be voltage and bottom wire is ground. Use digital multimeter on low voltage setting to check. The sensor sends signal to pcm, on the right in diagram, the dashed lines. Then the pcm sends data to instrument cluster on buss circuit.
I agree with other person, it could be low oil pressure and everything else is working correctly.

see this tips and fix it. God bless you
The oil pump supplies oil to lubricate your engine. If the oil pump is worn or is not turning, the engine will suffer a loss of oil pressure, which may result in engine damage or engine failure.
The first sign of trouble may be a low oil pressure warning light, a drop in the normal reading on you oil pressure gauge (if your car has one), or the appearance of ticking or clattering sounds from your engine.
As a rule, most engines only need about 10 PSI of oil pressure for every 1,000 RPM of engine speed. Oil pressure will read higher than normal when a cold engine is first started because the oil is thick. Oil pressure will gradually drop as the engine warms up and the oil thins out. So normal oil pressure on a warm engine cruising down the highway is typically 30 PSI up to 45 PSI.
SYMPTOMS OF OIL PUMP TROUBLE
The first thing you should do if any of these symptoms occur is to stop your car, turn off the engine, let it sit for a few minutes, then check the oil level on the dipstick. If the oil level is at or below the ADD line, add a quart of oil to bring the level back up to the full mark. Add as much oil as is needed to raise the level to the full mark. Then restart the engine. If the warning light remains on, or the oil pressure reading does not climb back up to its normal range, or the engine noise does not go away, you may have a bad oil pump.
The other possibilities include a bad oil pressure sending unit, or a problem with the oil pressure warning light circuit or oil pressure gauge.
OIL PRESSURE SENDING UNIT
If the engine is NOT making any unusual noises and seems to be running normally, and the oil level on the dipstick is FULL, but you are still getting a low oil pressure warning light or low gauge reading, the fault could be a bad oil pressure sending unit.
The oil pressure sending unit is mounted on the engine block. On some applications, there is a spring-loaded pressure-sensitive diaphragm with a switch inside the sending unit. This switch completes the circuit to the low oil pressure warning light if oil pressure drops below a certain threshold. The unit may stop working if the diaphragm inside fails, if the switch is stuck, if the small hole that allows oil to enter the sending unit becomes plugged, if there is a loose, corroded or broken wiring connector at the sending unit, or there is a fault in the wiring circuit between the sending unit and warming light.
On vehicles that have an oil pressure gauge (electronic, not mechanical), the oil pressure sending unit has a small rheostat inside that sends a variable voltage signal to the oil pressure gauge when the diaphragm moves. A worn spot on the rheostat or any of the other problems just described for the simple pressure-type oil pressure switches can cause a problem.
FORD'S FAKE OIL PRESSURE GAUGE
On many Ford vehicles that were built from 1980 through the 1990s, the oil pressure sending unit has two switches, a low pressure and a high pressure. These vehicles also have an oil pressure gauge, but the reading on the gauge is not a true indication of real oil pressure. As long as the pressure to the sending unit is between high and low, the gauge will read normal. If oil pressure drops and trips the low pressure switch, the dash gauge will now read low. Or, if oil pressure goes up and trips the high switch inside the sending unit, the dash gauge will read high. Consequently, don't rely on the oil pressure gauge for an accurate reading in these vehicles. It is only a gross indication if the oil pressure is low, normal or high.
OIL GAUGE PROBLEMS
If the engine is NOT making any unusual noises and seems to be running normally, the oil level on the dipstick is FULL, and you have replaced the oil pressure sending unit but are still getting a low oil pressure reading on the dash gauge, the fault could be in the wiring circuit between the sending unit and gauge, or the gauge itself might be bad.
Check the wiring connections on both ends as well as wiring continuity between the sending unit and gauge. If no wiring faults are found, hook up a pressure gauge directly to the oil pressure port on the engine and check oil pressure with the engine running. If the engine-mounted gauge shows normal oil pressure but the dash gauge is reading low, the problem is a bad dash gauge.
On the other hand, if the engine-mounted pressure gauge reads low and you have done all of the above, chances are the oil pump is worn, or it is not picking up enough oil because of a restriction or blockage in the pickup screen in the bottom of the crankcase.
OIL PUMP PICKUP PROBLEMS
The pickup tube has a screen on the end to prevent large chunks of anything bad that ends up in the crankcase from being sucked into the pump. But we are talking BIG chunks of debris, not normal wear particles or carbon or dust or other microscopic-sized abrasive particles that can cause pump wear over time.

spend the 12 dollars and get a universal mechanical oil pressure gauge, remove the oil sending unit on truck and screw new gauge into and start truck. Does it read pressure? I know people will say just replace oil sending unit but i like to visually check for a true oil pressure than to trust a light, at least you will know what pressure your motor has. if you have pressure then replace the sending unit. Most of the time when the sending unit goes bad your oil pressure gauge in the truck will run at 80 psi all the time. Since you have a light the sending unit just says it isn't getting pressure which turns a light on. So be safe and do as i suggest, if there is no pressure at the mechanical gauge then i hate to say it but you may have a bad oil pump. I honestly just think you have a bad sending unit,if you want just replace it and if lights go off thats good, but you still don't know what the pressure is and if you feel comfortable with this then do so.

You may have a bad oil sending unit this is located under the hood near the oil filter two wires are connected to it. i would change this and see if your problem persists, if so you could have an oil pump you wont hear anything until its too late. 20 psi is all you need to keep it alive. good luck

Try reseting the computer, by disconnecting the battery.

yes, I would not do anything as long as you do not hear any unusual noise in the engine ,

the oil pressure is always high when you start it,the oil is cold and creates a different reading.
a few things...to consider
oil pressure sender defective
too much oil
wrong oil type

The oil pressure sending unit is a sensor with an internal diaphragm, if the internal diaphragm is damaged , or the line to the diaphragm is blocked you can get this kind of fluctuation on the oil pressure gauge.

The oil pressure value must be read by installing a mechanical gauge near to the oil pressure switch and checking the pressure.

If pressure is ok, replace the sending unit. If there is a real pressure drop, check the oil pump.

If you still get fluctualtion after replacing the oil pressure sending unit, replace the oil pressure gauge.