I recently purchased a 2007 Pontiac Grand Prix 5.3L MFI OHV 8cyl with 150k miles on it and it is burning a quart or two of oil every 1000 miles. The previous owner told me this was the case, but said he had it into the dealer many times and they could never find the leak and there is never any oil on the ground. The oil is obviously going somewhere but would love to figure out where. Any thoughts?