You said "150 miles, 1 qt. oil" Do you think it leaked the oil (did you see any leaks)? Did it burn it? I realize that you stated that a guy following saw the smoke, but it's not uncommon for our diesels to burn a little.
If you could be a little more specific, it may help us diagnose or suggest some things to look for.
84 500SEL EURO 87K
85 300D 267K (and still goin' strong)