When first starting the 2000 jeep grand cherokee the oil pressure is fine, after driving 15 to 20 miles when you stop the oil pressure guage indicates the pressure dropping near 10 pds or less and the...
When the pressure reads low, are you hearing any knocking or tapping noises from the engine? If so, you may have worn engine bearings. I'd check the actual pressure by having a mechanical gauge installed on it in order to determine the actual pressure (your gauge is electro-mechanical and not quite as accurate) If pressure is actually that low the oil pan must be removed and bearings, oil screen etc need to be checked.
The reason pressure is always higher at first is that when cold the oil itself is thicker...as it gets hot and thins out pressure will drop somewhat, but should not go that low. Only other thing that can affect pressure in that way would be either a bad sending unit (you will know that after testing) or gasoline contamination in oil (happens if an injector is not working properly)
Good luck with it!
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