The symptoms can be pretty silent,
AFAIK, until things get REALLY bad - then power drops, it smokes black and/or blue, MPG drops, and oil consumption skyrockets. I don't think you're quite there yet. Bad turbo seals can suck a decent amount of oil but that's not terribly common. "Good", tight OM603's should be 4-5kmi/quart or better, in my experience. I'm not entirely sure how to test the turbo other than replacing it. I still doubt that's the problem. I wonder if a compression test would be helpful in this instance? Hmmm.
