What should a 2002 Pontiac Bonneville oil pressure gage read at

An old engine will normally show a lower oil pressure, which may be due to worn bearing, a worn oil pump or both.

A new engine with good bearings will normally show a higher pressure at revs than at idle, and a higher pressure when cold. Putting a worn oil pump back in a new engine (never a good idea without a thorough checkup on it) will modify this situation.

Just going on the readings I'd say your situation now is quite normal. You may even be able to change to an oil one grade less viscous, say 10W instead of 20W depending on your winter temps.

With an oil pressure gauge:
Remove the oil sender unit and install an oil pressure gauge at the cold engine and let the engine warming up. And record oil pressure data.
You will see if the oil pressure match your problems if so then there is an internal leak or bad oil pump or old engine oil.

Hope it helps.

A number of things could impact this, a dirty oil filter, different type or weight, of oil or quite simply wear within the engine or oil pump (or combination of both)
First I'd get a gauge under the hood and get a true pressure check on the oil, perhaps the sending unit is failing. If it reads fine be certain the filter is clean. If still nothing it is within the engine, you can try to use a heavier oil that may increase your oil pressure.

I hope thi may have assisted you,
Tom

It takes a second or two for the oil to build pressure. If it's really cold out , then longer. Spattering is normal.

Bad sending unit or stuck pressure valve in the oil pump.

The oil sending unit is on the oil filter adapter above the oil filter.You will need a special socket for the unit.Good luck.

Oil pressure is read by the sending unit, not created by it. If you replaced the sender and do not see a minimum of 20psi there is something wrong inside the engine.Some people will tell you that 5psi is "safe". In reality, in an otherwise perfect engine it may be but in real world application, the 20psi number is the lowest I'd ever accept in any engine.(at idle) Running pressure is influenced by temperature and oil grade, but this should not vary dramatically.
It is best to attach a mechanical oil pressure gauge to the engine and in that way, you can read the exact pressure. If the mechanical gauge reads the same or very close to what you see on the dash, then the reading is correct and if low you will need further diagnosis to determine why.

its probably the sending unit...cheap fix too.

Oil sending unit bad. Replace.

The oil pressure sending unit is located behind the oil filter, Threaded into an aluminunm housing. It has wiring that plugs into the sending unit. The oil filter has to be removed and you will need a deep well socket and a 6 inch extension. Change the oil and the sending unit together.