Recently my 1989 300TE was showing 1/2 bar at road speeds. Mechanic checked, 4 PSI at idle, 31 PSI at 3000 (hot)with 5W30 . Pressure erratic, he said it acts like a broken relief spring in oil pump - says probably need new oil pump. Not sure he can raise engine enough to get oil pan off - if can, says $2000. If has to remove engine, additional $1500. Car has 300K miles but has been well maintained and other than some play in steering drives well. Is new oil pump needed, and if so, does $$ seem right ? Is spending this amount worth it ? Would going to heavier oil help ?

Is this something that just started, or has it gradually gone that way? Coincidental to an oil change?

With your mileage, the cause could just as easily be worn main bearings. Given the expense and effort, I think I would look into en engine swap.

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Prost!

It may well be worn bearings, as they give low hot idle oil pressure and fair oil pressure at high rpms. I would try a thicker oil like 20W50 to see if you then get tolerable oil pressure at hot idle. A worn engine can go a long way with thick oil, and the thick oil will actually slow further bearing wear.

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'97 E 300 D

Could be the oil pressure sensor itself. Why don't you replace it at your next oil change? Cheap option if it is the problem.