I personally like Valvoline, use it in all of my vehicles. And I do run my 235 hard, too. The only thing I would do different would be to use straight 30wt instead of a multi-grade. The Chevy 6 seems to run happier on a staight weight oil.
About the only oil I would tell you stay away from at all costs is Quaker State. I've seen what Quaker does to the insides of an engine. Very nasty.
I will agree with gazim on frequent changes, every 3 months or 3000 miles, don't forget the filter if you have one. As far as additives go, you shouldn't really need any, but if you run anything, I would suggest either Marvel Mystery Oil or Lucas Oil Stabilizer. I've used both, and they do work.
I'm still trying to gather information on synthetics in old engines, seems some say yes and others say not to use it in an older engine. Right now, Royal Purple is the only manufacturer of 30wt synthetic, so that may be the best choice if you do try it.
Oh, and one more thing. For draining the old oil, get the biggest drain pan you can find. The smaller ones will just lead to the splatter mess that you unfortunatly had to experience.
