This is a rather open ended question and you'll get a myriad of answers which will all be valid to a degree. Brand comes down to personal preference or brand loyalty, I don't think one brand is particularly better than another. Mineral or fully synthetic is more about how much you want to spend unless you love running the engine really hot (and by hot I mean at the extreme end of the operating range) as synthetics are stable to higher temps but when you are talking about your average run about it's probably overkill. As for viscosity, follow the manufacturers guidelines for starters and then take into account engine life/wear etc
The most critical part is regular services, a mineral or semi-synthetic 20w-50 is a good safe bet of what ever brand you prefer. I like supporting local manufacturers where possible so Penrite oil is good for me, they also do oils with extra zinc which is good for older style engines like the 3800/ecotec V6 motors.
Just remember regular servicing is key.
