The recommended oil for the V6 ecotec is a 20W-50 rated oil, unless the vehicle is constantly used in snowy areas, then a 15W-40 rated oil should be used. You can of course use an oil that exceeds these recommended specs, such as penrite 15W-60 etc.
I now use mobil 1 5W-50 full synthetic type oil in my ecotec, the engines got around 200,000 km on it now..I used to use penrite 15W-60 semi synthetic but I kept having trouble finding a supplier that had it in stock all the time, so I changed to mobil 1 instead, which was available pretty much everywhere...