Fluctuating oil pressure gauge after 20 plus minutes of operation

If you check the oil pressure with a mechanical gauge and the pressure is steady, the problem would be in the gauge on the dash if you have replaced the sending unit and there are no problems with the circuit.
The mechanical gauge could verify a problem with the actual pressure in the engine.

replace the oil pressure senor they alway's fail on gm cars and trucks

More then likely the sending unit is faulty, but to be safe hook up a mechanical test gage at the oil pressure sender port and check oil pressure hot engine, if it is steady then replace the sending unit.

That is a good sign, it is working, my oil pressure fluctuates like a tachometer, at idle it is low, like 20 lbs, then when I 'm cruising, 35lbs, but it goes up and down when you pump the gas. The pressure gauge is reading the oil pressure at the top of the engine, it will show 45 lbs with only 1 qt of oil in there, it is not an oil level gauge. You might have an oil light buried on the dash somewhere that will light up if the oil gets too low. ,, Now about that coolant leak, it should have nothing to do with the oil pressure. Is it a hose? Radiator? Joint at the top of the engine? Where is it leaking, and what is it doing? Does it over heat? Or did it overheat? Is there oil floating at the top of the radiator? Does it blow steam out the exhaust? Does it have coolant or just water? Check it out. hope this helps.

Sounds to me like it could be a bad oil sending unit or gauge. I would take it in to a shop and have them test the oil pressure with their equipment.
If they tell you the pressure is good try changing the oil sending unit first then if that don't work, replace the gauge.

no,,, fluctuations mean =low oil pressure,oil level low,oil old or too thin viscotly,oil pump loosing or not develloping the correct pressure
pressure release valve spring weakened

The oil pressure guage will fluctuate with engine RPM. At idle it should be approximately 20 lbs./sq.in, at high revs about 50-60 lbs./sq.in. Hope this helps any concerns you may have. Good luck!

You'll need to determine if the oil pressure is actually changing, or if the sending unit or gauge is lying to you. Be certain the oil level is up to normal, then check the actual oil pressure with a test gauge. If the pressure varies (drops) at engine idle, perhaps your pressure bypass is defective, or perhaps the oil pump is worn. Testing with a known good pressure gauge should lead you in the right direction.

check powersteering hoses on passenger side. also may need to replace oil pressure sending unit

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.