Buick put diesels in the Regal during the early 80'*. My Dad special ordered a 1983 Regal V6 diesel and if I remember right, it was the first year offered. The thing got 32 mpg on the highway and actually got up to speed just fine. No turbo though.
I've never seen a diesel Regal since. Before that he bought a used 79 Cadillac diesel (350 converted). I remember shooting WD-40 down the throttle body every morning to get it started.