My E320's oil pressure suddenly dropped to .3-.5 bar at idle yesterday, with no perceived change in engine behavior. Changed the oil (15w40 diesel oil) and filter, added some MMO on the theory that gas in the oil had possibly gummed up the bypass valve--engine is also running rich due to EGR malfunction and possible vac leak. Pressure comes right up with revs, 3 bar by 1800RPM.
Took it out for an ITU after oil change (pressure at idle was 3 bar beforehand), ran great, but is now making a chirping/squeaking noise from the top end...sounds like maybe oil starvation to the valve train. Noise tracks with engine RPM and doesn't seem to go away with revs. Opened oil cap while running, splashes oil but much less than I remember my last M104 doing.
Oil pressure now about .5-.7 bar at idle, still comes right up with revs as expected.

Video of noise: https://youtu.be/M5vs_U4uO10

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)