There may be a restriction in the cooler system someplace. Funny I've never found anything better than grabbing the end of a hose or line and blowing through it, you can feel a restriction quick. Used this technique mostly on tranny coolers, but probably applicable to engine coolers also. I have on occasion found restrictions in various tranny cooler setups.
Air block may be a factor. If a bubble doesn't move, it eventually equalizes pressure with the shared medium (oil) and causes a restriction. It will eventually dissipate, but this takes awhile.
The 4.0 as near as I can figure from research (there isn't much info out there), oils from the top to the bottom. The cam gets oil first. If the top is noisy, this can be a sign the bottom of the motor may be oil starved.