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Usually means the engine bearings are worn out, or the oil pump is weak.
As the oil gets hot, it gets thinner, which reduces pressure. When the engine is at idle speed the pump is not pumping as fast as it is at highway speed.
It could be the oil pressure sending unit, or the engine could really have no oil pressure. If the oil pressure dropped to zero at highway speed, I would expect the engine to lock up or at least make alot of noise. You may have to tow it home and check the oil pressure with a mechanical gauge, or replace the sending unit to see if the gauge on the dash reads pressure. The fuel pump gets power from the pump relay at startup, then runs off power from that oil sending unit when the engine is running.
Oil pressure is always lower at idle speed.I'm guessing you have high mileage.If your oil light is not coming on in drive at idle with the engine cold,but it does when hot,your bearing clearances are probably getting a little loose.Try using one quart of Lucas oil additive in place of a quart of oil at your next change(or just change it now).This stuff is thick and stringy,it helps the oil cling to surfaces.(if there is no change after this,you could try a new oil pressure switch-it's cheap enough.I actually had the switch on my 95 dakota leaking a LOT of oil at high pressure/RPM,which affected the reading).
OIl will get thinner as it warms up and since the oil pump speed is directly affected by the engine RPM's, a drop in pressure after the car reaches operating temp is normal. The question is what are the pressure readings "cold" and "hot" and are they within acceptable specs.
At idle the oil pump is not spinning that fast. When your cruising down the highway, the engine is turning a lot faster and therefore the pump is spinning faster. The faster the pump spins, the more pressure it will make to a point.
Since pressure and flow can be affected by the "weight" of the oil and the operating temperature of the engine, It's hard to make a diagnosis on the internet. You should take it to a repair facility that you trust and have them hook up a mecahnical oil pressure gauge and monitor the pressures cold and hot at idle and crusing RPM's. Then compare the readings with what Jeep says is acceptable.
Low oil pressures can be an indication of among other things worn bearings.
Maybe your engine is idling too slowly, If the revs are below say 700rpm the low pressure warning light could come on especially when the engine is hot.. If the idle speed is ok try a heavier grade of oil in the engine. Next try a new oil pressure sensor. It's got to be one of these things. Check the engine running temperature . An over-hot engine will cause the oil light to come on too.
Sounds like you've got sludge. The best way to deal with it is to change the oil. Flushing with an oil cleaner is dangerouse but faster. The cause of this isrunning too long without an oil change, engine overheating, and antifreeze leaking into the oil. Lots of really short trips where the oil doesn't reach full operating temperture. Change the oil with a good cheap oil including the filter. Run the car for about a week 200 miles and change the oil again, filter and all. do this again untill when you go to change the oil it's not exactly black solid like the first time you changfed it. Another cause of sludging is the car does'nt warm up fast enough and not hitting the full operating temperture, it could need a new themostat. Overheating can cause problems too. Is the cheak engine light on? If it is you need to research the problems and get them repaired because everyuthing effects everything else.
Stop driving it until you have this checked out!
Low oil pressure is making the light come on.
The engine might seize up on you and that's big bucks.
Or course having the oil light come on may be big bucks as well. It means the bearings might be worn out and you'll either need to overhaul the engine or get a replacement engine - or new car!