When warm my 300E idles at just under 500rpm. While it is very smooth and quiet, my 250K mile engine has about .2-.3 bar of oil pressure when hot. The manual does state this is acceptable. But an rpm increase of 50-75rpm would bring that closer to 1 bar and make me feel better. I've read about the "purple" wire trick to fool the ECU to raise the RPM by maybe 50. But I've only seen early 86-89 cars referenced. Does this trick work on the 90 and up W124 too? Which have slightly different ECUs than the early models. Anything else I could do?

I'd be more tempted to pull the oil pan and install a set of rod bearings ( and maybe mains ) to tighten up clearances boosting oil pressure.

The oil pressure off idle instantly hits 3 bars by 1500rpm. Moving 50rpm with the accelerator pedeal moves it to 1 bar. Don't think there's any issues with engine health despite the mileage. I feel that 475rpm is too low for idling in gear when warm.

I had this exact same problem on my 84 500SEL (big difference I know) and it turned out to be the IAC valve. Cleaning it several times raised the RPM to about 550 rpm at idle.
Cleaning can't hurt so that is a good place to start.

Paul

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84 500 SEL (307,xxx miles)

Thanks for the response Paul. I did a few more hours of searching after my post and everything kept coming back iac/Icv. I think I've got a good spare somewhere. I'll try swapping them out to see if behavior improves.

I did the purple wire thing on my '91 300SE. I had the normal 600 RPM idle in gear, but it had developed an annoying buzz in the engine I was just unable to isolate and fix, and my hot oil pressure was dropping to around .4 BAR also. Cutting the purple wire from the gear selector to the ECM, and grounding the ECM side makes the ECM think the car is in neutral when it's in gear, and raises the idle speed to about 700. The little low frequency buzz went away, and pressure went to 1.0 BAR. The only negative is it does tend to tug a bit more at the brakes when sitting at a light. I think my in-town mileage dropped a bit too, from sitting in traffic.
But, as stated earlier, your engine should idle at 600 in gear, so the IAC or the air flow sensor may need work.

DG