I was wondering what some of you guys would recommend for a 2005 Chevy Silverado 5.3L Z71 with 179,000 miles. I have always used Castrol GTX but have wanted to switch to a synthetic for the longer OCIs. My driving is mainly short trips and city driving, with an occasional 300 mile highway trip every few months. I don't drive very hard or fast(mainly because of gas prices), but I have thought about a better oil due to the mileage. Any ideas?