Urban legend.

The gauge reads pressure in bar. Normal is no pressure with the engine off, so the gauge must read 0 with the engine off. Pressure varies between 0.5 bar and 3 -- the gauge pegs at 3 -- with the engine running. Less than 0.5 is a cause for concern (worn bearings, bad oil pump). Cold engine should run 3 bar til it warms up. Pressure must be 3 bar at 1000 -1500 rpm on a warm engine.

If the gauge reads 3 with the key on, engine off, either the sender is bad or the wire is loose.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!