Oil pressure should only be pegged once you step on the gas. Otherwise, while idling, especially in gear, it should be somewhere around .5 to 1 bar, depending on the weight of oil used.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280