Dexos is a specification GM has setup starting with most 2011 vehicles. To meet it, most brands will be semi-synthetic or full synthetic. To use the Dexos label on an oil product, the manufacture has to pay GM a license fee of $1000 and $.36 per gallon. A lot of brands would be able to pass the tests, but aren't going to pay to use the label.
5W30 Dexos blend. That part# is one of the gallon part #s that the dealers purchase for bulk oil under the GM Bulk Oil Program.