Buddy of mine has an XJ, with of course the 4.0 6cyl automatic. Lately once it has been driven for about 30-45min. the pressure drops from 40 down to about 20. He said the engine sounds good, no knocking or tapping like it would be starved for oil. He took the sender out of his wrangler because it has about 20 hrs. on it and it is still having the same results. Nothing seems different other than the gauge drops. It still runs great and has no noises. What else could it be? Do you think the motor is going in it? My 92 Cherokee did the same thing. At highway speeds (60-70) my presssure would be nice and high, around 40 I think, but after coming to a stop, at idle it would drop way down into the 20s. I never had an overheating problem with mine or any other motor problems.