Am still puzzling over this -- suddenly oil pressure went from 25@hot idle to 5-8@hot idle, and pegged (45) only at high revs -- town running shows about 30lb. I have not checked the gauge against another one because I do not have a spare & am not anxious to spend $20 for a gauge I'll use once for 1 minute as $$ is very tight. The dash gauge, however, acts like it's functioning normally.

I talked to a man with a lot of varied (but not MB) mechanical experience who suggested that my symptoms sound like a check or bypass valve stuck open. This seems to correlate precisely with what I'm seeing on the gauge -- that pressure takes longer to build up at all than it used to (seems like the filter housing is empty every time I start, like after an oil change), and falls much faster when the fuel pedal is released than it used to. And, lower total pressure would be explained by the pump's inability to build higher pressure with the valve stuck open.

The pressure relief valves and check valves seem to be both built in to the filter housing. Has anybody ever played with these .... or should I just try a used housing? (anybody have a spare housing around for sale?)