Thank you everyone for your helpful replies. The car has not lost its oil based on the dipstick. And there were no obvious new leaks or anything too suspicious under the hood. I didn't have much time today, but I removed the wire from the pressure sender. With it disconnected, and the key on (but no engine or starter running), the indicated oil P pegs (so that is a good sign that the gauge and wire may be okay). I didn't even get a chance to ground it, but my understanding is the indicated P should go to zero. In my research yesterday, I did find a reference to resistances I can install between the wire and ground, so verify if the wire and gauge are okay. Visually, the wire is a little beat up. The external insulation is cracked, but the internal colored insulation looks okay. I couldn't easily tell where the other end goes, but I think I want to avoid having to replace the wire if possible.

I really like the suggestion of putting a mechanical pressure gauge on the oil filter cap. I haven't yet figured out whether I will do a long term solution like that, or try to temporarily rig something to the port where the sender normally goes. But the bottom line is I will not start the car until I have a redundant pressure gauge installed. Thanks again for those suggestions.

Oh, and regardless of the outcome here, while poking around I noticed that the fuel line downstream from the prefilter had a leak. So I tugged slightly on it, and it was able to be easily slid off its connection at the pump. I feel so lucky to have found that. I know diesel is less flammable than gas, but I lost an old VW to fire years ago, and I would not want to risk losing another car to a major fuel leak.

Peachparts forum members rock!

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).