Well, I dropped off the E300D this am at my MB guy's shop. This guy has always been straighforward with me and I trust him, pretty much.

First thing he asked was what kind of oil have I been running. I told him that for the last 5500 miles I'd been using Pennzoil 15W-40 and before that I'd usually run Castrol 15W-40. He said "ah hah!" That he didn't like Pennzoil and that in this climate (Alb, NM) he was putting 20W-50 in his customers' diesels. That 15W-40 didn't stand up to this heat as well as 20W-50, etc.

Anyway, he's going to check the oil pressure and (hopefully) rule out things like a bad oil pump!