Synthetic provides better protection at start up among other things. The molecules do not break down as easy. There for they lube parts better and have a higher tolerance to heat and resisting breaking down as easy. Thats why you can go longer on synthetic oil than conventional.
Most of the wear on your motor comes from starting it up. When oil is cold it doesn't protect your internals very well. Synthetic helps combat that some what. Thats why its good to let your motor warm up if you can before driving it. The oil is designed to protect at the optimum temperature, because that is what your truck will be running at 95% of the time.