Oil gauge fluctuates back and forth. sometimes

Location of oil pressure sender unit depends on what engine you have. There should be one wire going to sender, and if it has come off, gauge will read zero. 4.0 inline 6 has sender low on passenger side of engine, above the starter.

judging logically by what you mentioned I would check the oil pump first.

Hi there:
It sounds like it could be oil pressure sending unit. I would check the connection on the sending unit. Below are the procedures for checking the sending unit, If it checks out good I would have a mechanical gauge set up to measure the pressure.

TESTINGTo test the normally closed oil lamp circuit, disengage the locking connector and measure the resistance between the switch terminal (terminal for the wire to the warning lamp) and the metal housing. The ohmmeter should read 0 ohms.To test the sending unit, measure the resistance between the sending unit terminal and the metal housing. The ohmmeter should read an open circuit (infinite resistance).Start the engine.Once again, test each terminal against the metal housing:The oil switch terminal-to-housing circuit should read an open circuit if there is oil pressure present.The sending unit-to-housing circuit should read between 15-80 ohms, depending on the engine speed, oil temperature and oil viscosity.To test the oil pressure sender only, rev the engine and watch the ohms reading, which should fluctuate slightly (within the range of 15-80 ohms) as rpm increases.If the above results were not obtained, replace the sending unit/switch with a new one.

Hope helps.

First we would need to know when this occurs. Asuming the oil level is at the full mark!. If the pressure goes up during acceleration and down at idle when engine is fully warmed up, then this is normal. An engine requires 10 psi of oil pressure per 1,000 rpm ( ex. 20 psi at 2,000 rpm, 25 psi at 2,500 rpm, and so on ) . If the gauge is moving back and forth erratically at idle , i would say the gauge is bad. If it drops to 0 while accelerating I would say your oil pump is bad. If from time to time it seems to work and other times it seems not to work, I would say the oil pressure sending unit for the gauge is bad. If the pump failed you would start to here a Knocking noise from the engine in a short period of time. Use the above scenario's to narrow down what it may be.

First we would need to know when this occurs. Asuming the oil level is at the full mark!. If the pressure goes up during acceleration and down at idle when engine is fully warmed up, then this is normal. An engine requires 10 psi of oil pressure per 1,000 rpm ( ex. 20 psi at 2,000 rpm, 25 psi at 2,500 rpm, and so on ) . If the gauge is moving back and forth erratically at idle , i would say the gauge is bad. If it drops to 0 while accelerating I would say your oil pump is bad. If from time to time it seems to work and other times it seems not to work, I would say the oil pressure sending unit for the gauge is bad. If the pump failed you would start to here a Knocking noise from the engine in a short period of time. Use the above scenario's to narrow down what it may be.

Oil gauge may be bad, you said you changed the oil sending unit, and tried a aftermarket which worked, so the other option is to have you oil gauge checked. The sending unit may be good but you gauge is not reading proper.

The most likely place for an engine to start leaking oil (when it's coming from the top and not the bottom) is around the valve covers -- the gasket goes bad. Next most likely is camshaft seals.

Sometimes just tightening the valve cover botls will stop it or at least reduce it.

I think I'd be real tempted to check connection at oil sender, for clean and tight, and if that doesn't help, for the price, I'd change it, but not knowing exactly which engine you have, I couldn't give you a ballpark on oil sender switch. You can look it up or call autozone, or similar for price.

Sounds like a bad pressure sending unit, or a partially plugged passage going to it. First though, check that the wire connection is good on the unit. Sometimes just adding a good additive such as marvel oil will free it up sufficiently to restore function. With no noises etc, that's the most likely source of your problem. If not, driving as you have been the engine would be junk by now!!!
Good luck

I'm guessing your engine is'nt making any tapping noise with the low oil press.
Your oil pressure sending unit is most likely the culprit.
Is your car smoking when the temp. goes up?
You engine could be air bound or thermostat could be opening too late (might need new one).
PLEASE DO NOT OPEN THE RADIATOR CAP WITH A HOT ENGINE.