All have to Remember that the Gauge Set Mercedes "Gifted" us with is A
"REPRESENTATION" of actual engine operations.(a La 1930s Technology)

The Real O.P. peaks FAR ABOVE 3 Bar.

If you were to remove the O.P. Sensor (A Screaming Bear to do,"For Fun".)
and replace it with an Actual Bourdon Tube Type O.P. Gauge fitting and line
to a Real O.P. gauge; you'd most likely be reassured.

That's the ONLY definitive way to tell...

Howeveah, with 300K ALMOST on the OM603 I'd be investigating...

Mos Likely , your Explanation of the Noted Differences with the New Sensor
is the Explanation.

I'd MAYBE try a Sump of Synthetic Rotella T 5W40 (just for grins and giggles)
and see if any diffenence.
(The synthetic Molecules are a little more Petite and Reaction might be Quicker.)

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'84 300SD sold
124.128