I think everybody is misreading the question.

I think his issue is that at hot idle he is pegged (so at least he's at 3 bar, but no way of knowing by the gauge itself how much above that).

Normal situation in a W123 is:

1) Cold start and idle, gauge pegged
2) At anything above idle (hot or cold), gauge pegged
3) At hot idle the gauge should be between 2.5 and 1.5 or so. Minimum specified at hot idle is 0.5.

At hot idle with a gauge pegged (so you have at least 3 bar, and who knows how much more), I'd say you need to investigate a little further.

The oil gauge in your 240d is mechanical, which means behind the gauge cluster there's an actual oil line feeding into it. Did something get pinched there?

How old is the oil? What viscosity are you running?

It could also simply be that your engine is idling too high, as idling a bit above normal idling speed would peg the gauge at 3 as well.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)