Originally Posted By: nap
Probably because the 5W20 is not Dexos while the 0W20 is.
Castrol's actually is. If GM is telling everyone to use the 0w-20, there's no reason Castrol shouldn't tell them the same. Of course, it makes sense to them to keep a dexos1 certified 5w-20 for those who will insist upon buying it. For AC Delco oil, that wouldn't make so much sense to keep the product.