Oil Pressure issues

[Oldman]

Sorry if there are similar posts here. I couldn't find on like this.
When cold, I have average oil pressure. Gauge is right in the middle. The warmer it gets, the lower the pressure reading is. I've tried several brands and weights of oil and no help. It's gotten to the point where I get a "DING" and a warning of low oil pressure when warmed up and stopped at a traffic light. If I put it in Neutral, the RPM's increase just enough to raise the oil pressure to a couple of lines above "0" and the warning light goes away. I've read all over this site that these oil pumps don't go bad, but I feel that I may be the exception to that rule.
If there's anything else I need to be checking, please let me know. When the car is warmed up at 70MPH the pressure barely reaches the 20 on the gauge. Pressure responds to increased RPM's, but not near as much when it's warmed up.
No other issues. Running great. Was considering selling it due to lots of small problems, but just ordered front struts, wheel bearings, fuel level arm contacts, ball joints, tie rod ends and am ready to bring her back to her glory days again. But without resolving this oil pressure problem - if there is in fact a real issue - I'm wasting my time.
What would cause oil pressure to be great with cold engine and terrible with warm engine.
BTW, I live in Savannah Georgia and we really haven't had anything resembling a winter yet. Highs in the mid 70's every day, so it's not any kind of freezing problem.
Thanks,

[CMNTMXR57]

1) Oil viscosity changes with temperature. Hence the different readings.

2) Common causes of low oil pressure are;

- Oil Pump wear
- Bad oil pressure relief valve
- Too much oil causing "aeration" (think of trying to suck up soda with a thick head of carbonation... Same concept).
- Plugged or dirty oil galleries. Not allowing what oil does get pumped up, to drain back properly. But this is more an issue at higher RPM, so I would rule this one lower on the totem pole as a possibility)
- Internal oil system leaks, say between the pickup and the pump.
- Plugged oil filter.

Saving the worst for last... Worn main bearings and rod journals...

[Oldman]

Thanks. I get get beyond assuming it's a bad oil pump, but everyone tells me these oil pumps don't wear out. I've got 285,000 miles on it, but the motor was replaced with a Goodwrench motor at 135,000 miles. Would it have come with a new oil pump? Would it make sense to drop the oil pan and check the pickup first, or just replace the pump and see if that fixes it. Obviously, I'll check the pressure relief valve first, since it's the easiest job. Oil has been changed 3x since this started, so it's not the filter clogged.
Thanks for the help,

[CMNTMXR57]

Kinda hard to know.

If it was a long block assembly, it doesn't come with the pan, which usually means the pickup and pump aren't there either, and needs to be swapped off the old engine or a new one put on.

If it was a complete assembly, which means it comes with oil pan, valve covers, LIM, water pump, timing cover, flex plate, harmonic balancer, etc, it is basically a drop in condition, then yes, it will have a pump and pickup.

I would always work from simple to hardest, so yes, if you're dropping the pan, check the pickup and screen for blockage, then move up to the pump.

[Oldman]

Thanks again, I'll report back when I figure it out.

[AJT2004]

Unless I missed it, have you tried a new oil pressure sending unit and/or cleaned the port it screws into? Also, have you verified oil pressure with an external gauge?

[Oldman]

Tried several sending units. Have not tried external gauge.

[CMNTMXR57]

Typically oil pressure sending units don't fail by having low readings. When they fail, they open the circuit and have infinite resistance, which would peg your gauge at maximum.