I have a 96 Discovery with 83K miles. The red oil pressure light comes on at idle after the driving for abour 20 minutes. When I rev it even a little the light goes off, but when it returns to idle the light comes back on. It does not make any strange sounds however I do not allow the light to stay on for more than a second for fear of damaging the engine. I've owned the vehicle for over 2 years and have not had this issue until recently. I just dropped the pan & cleaned the screen & tube, & put 10/40 in it for the summer. It looks like the engine was taken care of as there was no sludge to speak of in the pan. I'm not sure what to do next on this & don't want to drive it in this condition. I really like this vehicle & plan to keep it for a long time however I obviously need to get past this condition.I looked through the forum & can't find anything that states an oil pressure problem at idle only after warm up. Any suggetions on possible cause & what to do next? Your help is very much appreciated.

 

What is the idle speed after it has warmed up? If it is to low the oil pump will not turn fast enough to keep psi.
If your idle speed is ok then I would switch to a thicker motor oil. I would switch to a 15W40 or a 20W50, preverably a full synthetic.
If it just started with the warmer weather your oil is getting to thin for some reason. Internal engine temps to high maybe, but a thicker oil will cure that, then go thinner for the cold months.
Maybe your oil cooler is clogged.
Also a full synthetic motor oil will clean the engine as you drive.
Worse case your bearings are shot, but with only 83k I doubt it.

 

Stop driving it till you can get the oil pressure check. If the light is coming on you could be under 15#'s pressure and that will kill an old engine.