My brother has a 97 K1500 with the 350 Vortec, has over 250K on it and still runs great. Actually have to do the front brakes on it again this weekend and fix a couple of other minor issues. His is no where near as nice looking as yours is, his has been used an abused with lots of rust on the body panels. Frame is still in good shape though. I do all the maintenance on it and I've run everything from 5W30 to 10W40 in it without any issues, semi or full synthetic. Usually go around 5K miles before doing an oil change. Reality is these engines don't really care too much on what oil you run in them as long as you keep them full and don't neglect the oil changes. I usually put in whatever I managed to find on sale around here, which last time I did the oil change was with some full synthetic Pennzoil that I got from Menards for $17 for 5 quarts.