I don't know much about brand differences, I'd check out bobistheoilguy.com for that.
I can say a little about the 3800 though. The difference between reg and synthetic oil is so dramatic, so incredible that it's hard to put into words. I'd rather run used synthetic than new reg oil. GM could have saved themselves millions in warranty claims and earned a better reputation just by putting a 'synthetic only' oil cap on the car. I have a short list of making the 3800 the best motor ever made but that's another story...
THe 3800 seems to enjoy thin oil. Cars lube better and I made more on the dyno going thinner.