I would have been hoping it was the turbo, the other causes are harder to fix for the average DSMer. Which of the other two causes it is, anyones guess. Try a leakdown test and see if that leads to anything. Since you need to pull the head to do either one, you might as well fix both things. Get the head inspected and rebuilt, and you obviously have no choice but to use a new gasket. If it still smokes after that, good luck to you.
