[tbucketnut]

Welcome aboard, the help and people here are first class.
Jm, I recently did some research on oil and have found I like and use Shell Rotella T4 --10W30 is ok. I like the T4 because it has enough zinc in it. Oils for the most part are devoid of zinc these days because most camshafts have a roller follower, as you know the GMC mighty V6 has a flat tappet mechanical cam so zinc is what you are required to have to sustain good long life in the camshaft arena. One could argue that it is sufficient with having just the cam in a constant oil bath like this engine has but for 16 bucks a gallon for the T4 why risk it. I would stay with dino oil and away from synthetics unless you have new seals. My humble opinion only.
You could also stay with your favorite brand and buy a zinc oil additive too, you have options.

Steve