03 gmc envoy with 5.3 140000 miles. Oil pressure reading fluxuates between 0 and 40. When problem first started pressure read 80. When vehicle was new it stayed at midpoint (40) vehicle has plenty of oil. if you rev engine or increase rpms it goes to 0 and stayes there. let off and it jumps around like it did when started (0 to 40). No engine pecks while running. Display indicates low oil pressure and stop engine. orange light (check gages) comes on when at 0. any ideas. Service engine has not come on. thanks for any help

Adding fuel to the fuel tank is absolutely unrelated to to any change in engine oil pressure.
The oil pressure will be higher when the engine oil is cooler and lower when the engine oil has warmed up.

If you are experiencing abnormally high oil pressure on start up I suspect you are running the incorrect type and grade of oil in the engine. Only use the oil specification(type and grade) listed by the manufacturer for your engine and nothing else.. If you are unsure what that is then it is specified in your owners manual for the vehicle.

High fuel consumption has also nothing to do with your engine oil pressure. If you are experiencing high fuel consumption then please read and examine these factors..........................
Finding and Fixing the Causes of Poor MPG

Another gauge needs to be connected to the car to check that oil pressure sensor or the gauge itself is not the problem.

If an independent gauge shows the same readings I would suspect a sticking oil pressure relief valve.

Google search GM 5.3 oil consumption. This is a known and common issue.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

there is a book publish by the govnt or minister of transport on the exact milage as well if you know how many liters or gallons go in your tank like 80 liters and the gas is 1.00 a liter it cost $80 for 80 liters on a full tank you do 500 km but make sure proper conditions of 100km a hour tires all proper equal pressure the wind is also a facture there is a lot involved to calculate your gas proformance as well if you had a oil change done latly you can find out on a govt site for gas consuption or consumers report

Oil Pressure Test Check for proper oil pressure at the sending unit passage with an externally mounted mechanical oil pressure gauge. A tachometer may also be needed, as some specifications may require running the engine at a specific RPM.

1. While the engine is cold, locate and remove the oil pressure sending unit.
2. Following the manufacturer's instructions, connect a mechanical oil pressure gauge and, if necessary, a tachometer to the engine.
3. Start the engine and allow it to idle.
4. Note and record the oil pressure reading when the engine is cold. The engine may need to run at a specified RPM, if so check the vehicle's specification.
5. Run the engine until normal operating temperature is reached.
6. Check the oil pressure reading again with the engine hot and record the value. Turn off the engine.

Compare the hot oil pressure reading to the specification. If the reading is low, check the cold pressure reading against the specification. If the cold pressure is well above the specification, and the hot reading was lower than the specification, the engine may have the wrong viscosity oil. Change the oil, making sure to use the proper grade and quantity, and then repeat the test.
Low oil pressure readings could be attributed to internal component wear, pump related problems, a low oil level, or oil viscosity that is too low. High oil pressure readings could be caused by an overfilled crankcase, too high of an oil viscosity or a faulty pressure relief valve.

Go to www.reliable-store.com and get all info on whether to repair or not, service it or not, and how to repair etc

No problem with the oil pressure, the norm is between 40 to 60. Anything lower then 40 you have low oil pressure and anything above 60 then you have high oil pressure which is caused by a blockage in the system.
What is common is the oil pressure gage fluctuating for the first 200 miles if the oil pressure sending unit is getting worn. Good maintenance will keep the engine clean and running good but the oil pressure sending unit is inexpensive and easy to replace. Next time you do an oil change you can have them change the sending unit.
Good luck and hope this helps. You can pick up the sending unit at Auto Zone and most auto parts store and will have the step by step to replacing it. If you do attempt to replace it. make sure you DO NOT USE any Teflon tape or silicon on the threads, they are tapered and need a good ground to the engine for proper readings.

Sounds like an oil pressure sensor. Here is a diagram to locate it for removal. Best of luck.
Greg

http://www.chiltonlibrary.com/content/images/21202/images/21202_GDIA_g518.pdf

could be one of two things, oil pressure switch (located on top of engine behind intake) or instrument panel cluster, if your vehicle has under 70,000 miles, it's covered under warrenty

That is probably a defective oil pressure gauge or a broken oil pressure sender, it reads 80 psi just because that is the maximum value allowed.

Pressure can be verified by reading it with a gauge tool.
If you really have pressure over 70 psi when engine is running then start replacing the oil filter, and check that the oil lines from pump are not obstructed, if there is nothing in there then replace oil pump.

If instead the gauge reads 80 with engine off, that is a defective pressure gauge.