We do not put Synthetic Oil in an Engine until it is well broken in. We reccommend that you do not change the oil right away, put 25+ hrs on the break in oil. Your first oil change should be 40W Castrol then after 75+Hrs or when the engine is well broken in you can change to synthetic. We have seen many problems with putting synthetic oil in your engine too soon. Such as the rings not seating. Engine uses oil. Ect.
I will call GM Monday to double check oil reccommendations and oil capacity for you.
The part number listed is a 7qt oil pan in the book but I will see what they say on Monday and let you know. I will also ask them what oil they suggest for your location. (could be different)