My 2003 Impala with the 3.4L engine (128,000 miles) specifies 5W-30 oil in the manual (see below), and I am currently using Mobil 1 5W-30. I see no reason to vary from the recommended oil weight. Startup rattle is minimal, and seems to have improved over the years for some reason. I don't think it has ever had anything but 5W-30.