You brought back an old, sad memory. I hope you don't go through the same thing I did. I also hope one of the long time MB Techs can shed some light, just to satisfy my curiosity, and more importantly fix your car. I hope you don't end up going through what I went through. If I would've had mercedesshop back then, maybe I wouldn't have had the sad experience.
I bought a really nice, stick shift, '68 230 sedan, cheap. Cosmetically it was like new and the entire car minus the motor was near perfect. It was fun to drive. I bought it with the full understanding I would have to overhaul the engine, low oil pressure, high oil consumption.
I took it apart and things weren't that bad, cylinder wear, crankshaft wear, etc. I replaced rings, bearings, oil pump, valveguides, valvejob, the whole bit. The camshaft journals and lobes lubricated via a spray tube that ran the length of the camshaft. There was one journal of the camshaft with some clearance, but it was lubricated from the tube, not pressure from below.
I got it all back together and it ran really nice. And decent oil pressure when cold. But when it warmed up, the oil pressure would go near zero at idle and to zero when starting off. I replaced an oil pressure regulator, even put a second head gasket on it, because of an oil passage at the front of the head that I thought may be the problem. I talked to an old time MB mechanic, and he never could give me any suggestions.
Generally, low oil pressure indicates worn bearings. I've seen alot of people think they could cure it by replacing the oil pump, but I don't remember this ever curing the illness. Of course, there are alot of places in the engine that if not right could bleed off pressure, and every model engines lubrication system routes the oil differently in some way.
I think that your 280 engine is a close cousin to my old 230 six cylinder. I hope that we can generate some dialog with the experienced MB guys that have been around long enough to have a history with these engines. I would like to satisfy my curiosity and more importantly, not see you go down the same road I did. I ended up getting rid of an otherwise really neat Benz.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in