Synthetic 5w50 Schaefer oil

Where you live I think I would use a decent normal oil of about 10/30 -10/40 range if it were my truck and an LA motor.
5.9 I use full synthetic 5/30 because those motors are made for synthetic and it isn't all about pressures.
A truck with generally low speeds and higher temps can benefit from a fast running oil that helps carry off heat and the synthetics do that.