My dad has a 2003 chevy silverado 1500 with the 5.3 . He has used Mobil 1 5W-30 since it was new with a wix filter. As many of you know this engine has the "piston slap" issue. I would like to stick with a synthetic if possible. The truck is at about 125,000 at present. He lives in seattle so it sees some cold weather but nothing too extreme. Your Opinions are appreciated!