Bringing this back up. I never believed the people saying 30+ MPG on the W123s. Got 31.4 MPG on my 1985 300D Turbo saloon with freshly changed Rotella T6 5W40, new tires, and a recent tune up. Also probably helped I was driving on a county highway with a speed limit of 60 mph, maybe 25% city driving. This engine still sputters a bit on startup (valve stem seals?), and is by no means perfect. But just replacing the 15W40 dino with 5W40 synthetic was enough.