Engine used quart of oil/500 mi. GM standard is a quart/1000 mi. R&R engine cradle, disassemble engine to find bad piston rings. GM had a Tech Svc Bulletin in 04 about this problem. Turns out that oil had been getting past rings into combustion chamber. Otherwise, pistons & bores looked perfect. Cleaned carbon off valves & piston tops & heads, replaced rings, broke in starting at 50mph, & increasing by 5mph every 100 mi. Now she doesn't burn a quart of oil in well over 6000 mi - I haven't pushed it to see how far it would go. Major pain, required long use of rental/loaner, not to mention all the expensive oil I had been burning. $4000 job paid for by ex. warranty. I had to pay for misc. filters & new lubricants. Now using Mobil 1, and fantastic oil mileage continues at 23000mi on the job. Looking forward to many years of 'new engine' use & performance.