What could be causing this?

It is possible that the low oil pressure when idling in your 94 Dodge Dakota 3.9 could be caused by either a faulty oil pump or a faulty oil pressure sending unit. It is difficult to accurately diagnose the issue without further information and a proper inspection.

Your engine is worn, likely from insufficient oil changes. This causes the oil pressure to drop at low RPM's and causes the warning light to come on.

That means low oil pressure,usually oil pump,head gasket or not enough oil,faulty oil pressure sensor even low idle could cause that

500 rpm when not in gear seems a bit low but not ridiculous. Oil light comes on when oil pressure is too low. Is your oil very old, do you fail to change it ever. Or your idle speed just isn't high enough to maintain pressure. You may need a new oil pump.

Sounds more like an engine idle issue than a oil pressure issue. If the engine idle drops too low, the oil pump cannot provide sufficient oil pressure. If there is no oil light when the engine is idling normally, then there is no problem there.

this means a mechanical gauge test is required to check actual pressure warm idling if low below 20 psi then the oil pressure pump relief is stuck open or engine bearing are worn

Mystery oil. Find at advance auto, red and black can poor into engine oil

This is because the oil pump is not pumping enough oil. It is either very low on oil, or the oil pump is going out. The light comes on when the oil pressure drops below 5 psi. This is very low. It should run about 25-30 psi at an idle.
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this is a common sign of a bad oil pressure sender, but could also be caused by low oil pressure, a sender is cheap,but it"s always smart to do a pressure test first with a mechanical gauge,you can obtain a mechanical gauge from most automotive parts stores. then you"ll have to locate and remove the oil pressure sender and install the gauge you bought. make your test at warm idle you should have at least 25 psi below that you may very possible have worn engine bearings making it impossible for the engine to have any better oil pressure at idle. if you have 25 or greater psi of pressure at idle replace your oil sender and you"ll be back on the road

If the idle speed drops to a low enough point when the oil is hot, then the oil pressure light will come on because the oil pump, which is driven by the engine, is not turning fast enough to maintain proper pressure. You'll need to find out why the idle is dropping and almost dying. It could be a bad IAC valve, dirty throttle valve or something like a vacuum leak. It would be helpful if you had a scan tool to monitor IAC (idle air control valve) counts as this is happening as it would help you find the cause. This may be a job for a shop, but don't let them sell you an oil pump until the low idle problem is fixed.