montanaman said:
Here are some numbers for the larger filter size:
Napa: FIL 1515 (this is a wix filter)
Wix: 51515
Motorcraft: FL1A
Basically, it's a standard-size "Ford Filter," and it adds 1/2 quart capacity to the Jeep 4.0L motor.
It's good to have a larger filter, but I don't think it will increase your idle oil pressure.
And the equivalent K & N filter is an HP-3001 (that is what I am running now on my modified Renix, 87. It fits the newer, 1991 and newer model 4.0s). These filters hold about 1 quart of oil, as montanaman said they hold twice as much as the stock 1/2 quart filters.
The larger filter added about 3-5 psi to my hot oil IDLE pressure, and about 7 psi to my cold and warm oil idle and Cold to Hot cruising oil pressure. I had used Quaker State, Fram and Mobil-1 filters in the 1/2 quart sizes, and just upgraded to the K&N 1 quart filter last week. It did make a difference! The larger filter will also last longer before the causing a pressure drop across the media as it will hold twice as much dirt as the standard filter. Dirt that gets filtered out will slowly cause a pressure drop through the filter and the oil pressure sensor is reading the pressure after the filter! Stay away from Fram filters. Search the terms "oil* pressure filter" for more details on what filter brands to avoid!!!!!
First, as others said below, checking the real oil pressure with a guage is best to do first. Then if the gauge is wrong, replace the pressure sensor/sender near the filter and clean the electrical connector that attaches to the sensor. They do go bad!
Based on your idle and cruising pressures it sounds like you have some engine wear, but in my opinion not enough to start doing engine work yet, especially if hot cruising pressure with multi-vis 10W30 is still giving 40-45 psi. (By the way it sounds like the pressure sender is working if it reads 40-45 at cruising speed, and lower readings at idle)
I have 246,000 miles on mine 87. Mine got down to 10 psi about 7,000 miles ago using10W40 Wt oil. It has had loose tapet noise for over 25,000 miles, since I bought it 2.5 years ago, that comes and goes (noise at start up, goes away once it is hot).
The factory spec idle pressure minimum is 13 psi, and considering sensor and gauge precision issues, (and possible low idle speed?, how is the idle speed????*) I would not panic yet as the gauge system reading error is easily +/- about 3 PSI in my opinion. 40 -45 psi at cruising says you are still pretty much OK, in my opinion.
In my case I have gone to a higher viscosity oil (went from 40 wt to 20W50) in the winter and I also add a quart of Lucas oil in the hot summers (gets to 100 F here) to keep my hot idle oil pressure up in the range of 20-30 psi now.
*Also, I have noticed with my Renix 87, 4.0, that there is about a 7 psi pressure oil pressure difference between about 500 rpm and 700 rpm idle speed, and a TPS calibaration voltarge change of about 0.10 volts can raise (or lower, -0.10 V) the idle speed that much.
If your gauge/sensor is correct and the idle speed is OK I would switch to a larger filter, and a higher viscosity oil and if need be add something like a quart Lucas oil. Do what ever combination it takes to get that hot idle pressure up over 20 psi.
If at some point you ever pull the oil pan to replace the rear seal, gasket.....check and replace any bearings that are showing signs of wear and upgrade to a high capacity oil pump while the pan is off.
Let us know what all you find out!