When a turbo starts to go it will start useing oil and blow it right out the tail pipe, When one goes all the way I have seen them suck every drop of oil out of the engine Lots of black smoke and you had better get it fixed soon. If it is valve seals or Guides it will puff blue at start up because the oil rins down past the seal or worn guide and right down to the valves theres your blue smoke . now a valve cover can leak oil but really two qts in 400 hundred miles is pretty extreame, Diesel leak oil every where but usely large amounts mean turbo, or some other major leak from lines, filters, turbo feed and return line, oil pans are nortorous along with front seals and real seals, so what I'm trying to say here is black smoke means turbo , but only from my experience, Jerry Master certified ASE mechanic