well, putting good oil in a crapy gmc is asking for problems. but, i'd question if the engine was bad in the first place, not if the oil caused the problem. Changing the oil on time is important, but i don't do it my self, i take it to toyota. they will put in synthetic oil, and it does meet toyota's specs, so there's nothing wronge with it. From my understanding though, the main difference is when you drive the car a lot or hard, or if it has high millage. It's suposed to be better on the engine, If it was worse, toyota wouldn't use it or recomend it.