GM's oil suppliment isn't that important if you stick with the stock springs. There is no proven reason not to run synthetic in a break-in motor except that you will be changing your oil early and it's a waste of money to do for that reason. Do keep the rpms up, don't lug the engine while driving, no prolonged high rpm or high power output, and don't maintain speed. You should burn some oil within the first thousand miles (less than 1 quart). Afterwards, your oil consumption will stop and your motor is broken in. I ran Castrol GTX 10w30 for my break-in. Changed the oil at 1200 miles, 4500 miles, and now I'm at 9000 miles. If you look at the API grade on the back of the oil, most oils are SM grade. SL grade would be preffered for flat tappet motors because of its high zinc content. Some of the heavy duty oils such as Chevron Delo 400, Shell Rotella, and Mobil Delvac stll have the SL rating, for now.