I'm kinda taking over the oil changes in my dad's '05 Chevy 2500. He bought a newer F150 that he got a good deal on so the Chevy is kind of a shop truck now. It has a 6.xL gas engine I think. It is used in the winter months for plowing snow and just a lot of short trips/hauling otherwise. He always put 15w-40 in it for some reason so I'm going to start using the 5w-30 like it calls for. Right now I'm using Service Pro semi-synthetic 5w-30 but was thinking of going with PP for the next change. The service manual doesn't really offer what the change interval should be either. It only talks about waiting for the 'change oil' to go off and change within like 600 miles or something. Is that the best way to go or should I do like 5000 miles/200 hours or something like that? The way its used, I'd rather go by hours than miles especially since it measures hours already.