If the truck used regular oil in the past, use the high mileage oil. If the truck used synthetic oil in the past, stick with synthetic oil. What does the manual say for oil weight? In the winter use 10w-30. Try that weight in the summer, and if it uses oil, switch to a higher weight. In the past, I've used Castol 20w-50 in old cars, but the newer cars have tighter clearances. If it smokes, and uses a lot of oil, consider Lucas oil additive.