I wish I knew the precise answers, guys. This truck is new to me, I have only changed the oil once. But it was dealer serviced diligently... from the records I have it looks like a simple 3k-5k OCI was kept with whatever 5W-30 the dealership had on the shelf. The records have Castrol GTX mentioned, synthetic blend (no brand stated), and Exxon. Whatever they did, it worked! Doesn't burn a drop of oil from what I can tell.
I've since moved to Valvoline synpower which I'll change around 5k.