Do I have a serious engine problem or is it the oil pressure gauge?
 

!985 380SL, 240k mi., the motor feels strong

While cruising at 40 or so MPH the oil pressure reads 3 bars if I let off the accelerator it may drop to 2.75 or so BUT as I come to a stop the pressure drops to 0 while the idle is abound 5 - 700 RPMs. I usually shift to neutral, the revs increase and the needle bounces between 1 and 2. Should I have the car towed to the shop?

Thank you for any replys

what weight oil?

Well, it sounds like the gauge is actually doing something, since the indicated pressure comes up when the RPMs increase. But if it were mine, I'd connect a mechanical gauge to the engine to know for sure. Electrical pressure gauges are hard to trust.

20w 50 and I had a recent oil/filter change

I would not fret. Mine does this as well when the engine is fully warm (around 85°C). Its at "3" when normal driving, but as mentioned when the engine is fully warm (but only above 85°C), it will drop down to "0" when idling in gear, but in park, it goes back to about "1.5". I was told it was not a big deal as long as the gauge was back at "3" by 1800 rpms, which it is. My MB tech said it was OK. Its been like this since I have owned it. The PO said it was like that to when he bought it.

Where my thermostat is bad, the engine runs cooler... so its been around ".5" when idling in gear, with the engine temp around 80°C.

Too low for 20 W 50 oil -- you may have a bad seat or ball on the pressure relief valve, or a bad gauge. Easy to check, have your tech put a real mechanical pressure gauge in place of the sending unit.

You really need to have 0.3 bar minimum pressure at idle, else extended idle will cause the main bearings to fail.

Real pressure that low indicates a leaking pressure relief valve in the oil filter housing, worn main bearings, or a worn out oil pump.

Peter

I agree on it being too low for 20W-50.

Mine does that now with 5W-40. It would bottom out at .3 with 15W-40. For a day or two last summer, when I first noticed the issue and added 20W-50, it never dropped below 1.

Oil pressure
 

Mine recently started doing the same thing after an oil change, I re-changed the oil to no avail. I can see oil flowing on the valves fine but the pressure reads zero at rpms below 800 or so. Thing is: I can't find the sensor. I have a 1985 500sl euro.. Is the pressure relief valve available separately??

Oil Pressure-
 

Check the Oil Pressure Sender Regulator. Very common failure- Did 2 on my old 300E.

Where is the oil pressure sender???
 

On my 1985 500sl euro. Can't find it anywhere. I'm almost to the point of following the wire.

DRR

I wouldn't run it until you hook up a mechanical guage to either prove or disprove the readings. Do you notice any top end sounds when its reading 0?

Minimum spec is .3 bar, 0 bar at hot idle will ruin the top end in short order.

No top end sounds when reading zero, do get some valve train noise right after oil change until pressure builds then none, and after the temp gets over 80 I get the zero reading at idle in gear. Where can I hook up direct reading gauge if I can't find the sensor?
If it is the oil press regulator, my book says the engine needs to be pulled to get the pan off, UGGGGHHHH!