After I put 4k miles on my new 2003 Mercury Gran Marquis, I changed the oil to a full synthetic 5w-30. I didn't feel comfortable using 5w-20 oil in this Texas heat, especially driving in stop & go traffic every day. The car now has 23k miles on it and so far no mechanical problems that I'm aware of. The car has never used any oil between changes. So far so good.