Originally Posted by Fortune
I picked up a new to me '98 TJ with 123K miles.
On the 600 miles drive home, I once noticed the oil pressure had dropped to 20 psi. I had been running 40.
I stopped for gas and it was back up to 40.
That is not a problem, in fact that is excellent oil pressure and is exactly how it should be.
Oil pressure is generated by a mechanical engine driven oil pump. The faster the engine turns, the more oil pressure is generated... so a lower oil pressure at idle and a higher oil pressure on the highway is normal and the way it should be.
A good rule-of-thumb for acceptable oil pressure is 10 psi per thousand engine RPMs.
Because too many people were unnecessarily alarmed by the TJ's oil pressure gauge showing the true oil pressure and it's normal ups and downs, Jeep engineers dumbed the gauge down in the early 2000's so it only shows a mid-scale indication if everything is ok. This is a phenomena that has only happened since cars stopped coming with oil pressure gauges like they used to. Not many people still know that oil pressure is supposed to vary up and down with engine RPMs.