I almost lost my 4.0 to a fram filter (on the way to a weekend of wheeling no less). With no warning I noticed that I had 0 oil pressure about an hour into my drive. I was figuring that my pump was going (didn't know about frams yet) and my "fix" was to stop by a parts store and refill the truck with a heavier weight oil. that caused the pressure needle to at least move, though it never got above 10 psi when revving the engine.
After babying it the entire weekend, I got home and asked around. A couple guys mentioned the filter and so I swapped it out. sure enough, the pressure came back immediately. Never used a fram since.