All GM cars say to not use 20W-50 and 10W-40 oils. But it says you can use them if in a foreign county if the preferred weights are N/A, 5W-30 or 10W-30.
It probably is CAFE, the auto manf's are required to advise their customers to use these oils. Also I believe there are viscosity additives in some 10W-40 oils that make them undesirable for their engines, ring material incompatibility over extended miles. I read that somewhere in some GM literature but darn if I recall.