had the same type of symptoms on my wife's diesel VW 1994 Jetta. We bought it used and it used 1 quart every 200 miles when we got it. Never saw any blue or other color exhaust smoke whatsoever. Turned out to be a scored cylinder wall in #3 cylinder. Honed the cylinders and rebuilt the engine and have put 100,000 miles on it and it only uses 1/2 quart between oil changes (5000 mi). The thing to remember is that diesels burn the oil that leaks into the cylinder because of the higher compression so smoke may not be visible like on a gas engine. Also, no spark plugs to foul and make engine run rough. Even with the scored cylinder wall, my wife's car ran perfectly - but chewed the oil mercilessly. If you did not check the roundness of the cylinder walls before you installed new rings - that could be your problem. Hope this helps