From Don in the TECH TIPS...
"In the event that you might be facing oil pressure issues that do not respond to normal adjustment (35 to 40 psi fully warmed up at normal cruise, and 20 psi or so at idle), you might want to consider the regulating valve found in the overhaul section of our online catalog before taking on any heavier maintenance on your engine. Many overhauls have been headed off by the installation of this “early model” style of valve.
Another significant aspect to the problem of regulation relates to the fact that the orifice in the block is not always perfectly centered with the centerline of the threaded shaft.
The effect of this misalignment sometimes causes oil pressure to take a nose dive during an adjustment, as the threaded shaft nears the end of its travel and forces the ball (or even the pointed shaft of early model valves) off to the side of the orifice in the block. In these cases, it’s usually necessary to use the regulating valve seatdressing tool shown in the specialty tools section of our online catalog to bevel the orifice slightly, bringing it into alignment with the shaft or ball."
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HERE