I'd be more suspicious of the oil pressure sender failing. Every 603 I've had has done this and a new sender always rectifies the situation. Every one of them behaved exactly like this one. 3bar at cold start, then random dropouts or periods of "0" oil pressure once warmed up.

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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]