Mercedes requires that the warm engine idle oil pressure exceed 0.3 bar and the the oil pressure exceed 3 bar at 3000 rpm. As long as these conditons are met using oil that Mercedes approves for the conditions, oil film strength will be sufficient to keep engine wear to a minimum in a properly functioning engine.

My information sources are the Technical Data Passenger Cars (5/'85 & 12/'88) publications.
Those number will be appropriate with any approved oil that meets the viscosity and temperatuere standards. In truth, when at operating temperatrure, none of my engines ever drop below 1 bar under any conditions and all reach 3 bar by 1500-2000 rpm. That's using Mobil 1 15W50 or Delvac 1 5W40 oil.

Reports I've gotten from owners using Mobil 1 0W40 suggest that the oil pressure numbers are a tad lower and the needle pegs a few hundred rpm higher (but WELL below the 3000 rpm limit).

If the "O" rings (MB # 015-997-94-48) on the oil filter cap stem have gotten VERY hard (they get hard after about 25kmi in my experience) the oil pressure will be lower at idle and rpm will need to be several hundred rpm higher for the needle to peg. If you have hydraulic lifters (OM60x engine) the lifter may be noisier when the "O" rings leak. If the seal within the filter is displaced when installing the filter OR if the filter collapses, oil pressure may never reach 3 bar and the filter will need to be replaced promptly or engine damage is possible.

Marshall