Hi there,
Does your 280 have a vacuum pump mounted on the front of the engine? The reason I ask is that I once had a 280 with a vacuum pump that smoked like that, and the only thing wrong was the vacuum pump had a hole in the diaghram and was sucking oil from the engine. I just removed the pump and replaced it with a plate, as the gas engine makes plenty of vacuum to run everything without the pump. As I recall, the #1 cyl. plug got black with soot also as the vacuum pump was connected to the front runner on the intake manifold.
Richard Wooldridge
'82 300D/4.3L V6, over 60K since the conversion, no probs so far!