Lots of talk in this thread and others in the past on the increase in oil pressure being one of the main indicators of a good filter. If fact, it appears that many have changed to Mobil 1 and Purolator filters to resolve or help with low pressure problems.
After a close look at the 4.0's oiling system, it appears that oil flows from the pump, past the oil pressure sending unit to the filter and then on to the main oil galley which then feeds the crank, cam bearings, top end, etc. The important thing to note is that oil pressure is measured between the pump and filter. As mentioned in mudpuddle's response to the comment on blow-by: "Oil pressure does not indicate a particular filter's ability to filter, only it's ability to flow. A higher pressure (new) may indicate a restrictive filter."
A filter such as the Mobil 1 that is known for it superior ability to capture microscopic particles will show an increase in oil pressure at the guage, but the pressure drop across the filter of 6 - 8 psi, results in less pressure available at the cam and crank. Ditto for the Purolator and K&N; good filters, but they may be too good.
I've used Purolator in the past. On my '90 XJ with 250,000 miles, it was typically good for an indicated 10 psi improvement. Once I took a close look at the oiling system, I found that the extra pressure wasn't doing the engine any good. Looking at the web study on filters, I switched to a Wix filter. It does a good job of filtering without the restriction of some of the other high end filters. I think the Wix results in a 1 - 2 psi pressure drop. It also has a good anti-drain back valve and quality construction.
If I were running Mobil 1 or another brand of synthetic oil, I'd use the Mobil 1 filter. But on an older engine where oil pressure is an issue, I'm sticking with Wix. I don't want to leave half the horses in the corral.
