I maintain my father's 2002 Northstar equipped Caddy Deville. The car has 8K miles on it. Manual calls for 5w30 I change the oil (7qt capacity) at 1K, 3K and 5K with Chevron 10W30. No noticable oil consumption. The old man wanted to go synthetic so I started using Mobil 1 10W30 at 6K. Now at 8K I noticed he used about 1 quart of oil. I suspect the Mobil 1 is too thin for the likes of this motor. I'm thinking at the next change I will try GC 0W30 or Mobil 1 SUV 5w40 (Delvac 1). Anybody have any experience with oils that work well in this motor? Thanks.