My parents had 2 of the GM diesels. Horrible problems with wrist pins. First car ('78) they replaced the engine as it failed with 80k miles. Second car they gave us the engine but we had to pay to have it installed (120k on it, 100k was dividing line). But by that time they had it mostly right as the car had another 150k on it when we retired it. But being NA it made no power. Olds used a 3 speed auto and it would get high 20's mpg. Cadillac used the same engine with a 4 speed auto (OD) and it would get high 30's mpg.