Just make sure ALL fluids are changed. Not only the engine, but also the transmission and differential. And for 4X4, change both differentials and transfer case. Each can have their own unique fluid depending on the vehicle. Many people never change these. Then when the vehicle gets high mileage, it has problems. I've seen some transfer cases require regular car oil. I've seen them require transmission fluid. While I've seen others have a specialized fluid at nearly $60/qt that you can only get from a dealer. So it all depends on the year, make and model vehicle. Hopefully you can put the truck on a lift and check out everything thoroughly.