What kind of RPMs are you idling at? Does the oil pressure rise when you raise the engine RPM? You are running the correct weight and quantity of oil? Be aware that the oil pressure gauge in the W126 is electronic and the sender is known to fail outright or go erratic with age.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]