Let me further my post above with a continuing thought. dexos1 has created a new niche in the oil market for a product that is a little bit "better" than prevailing ILSAC and API specifications. But it's not as rigorous, either, as something like ACEA or any of the manufacturer specifications (BMW, VW, Honda HTO-06, etc).
I think the oil blenders, and Shell in particular, saw that as an opportunity to add a fourth major position to their portfolio. They have conventionals (QSGB, PYB, FS). They have high mileage offerings (I include Defy in that). They have "full synthetic" offerings (QSUD, PP, etc). And now this dexos1 thing comes along. They had what I believe to be a very slow-selling Pennzoil Synthetic Blend product on the shelf...competing for space with their better-selling products. Shell probably saw three options here:
(A) Make any adjustments needed to PYB (for example), get it certified, and then market that oil as a dexos1 offering.
(B) Many any adjustments needed to PP (for example), get it certified, and then market that oil as a dexos1 offering.
Neither of those options gain them any shelf space. (A) would probably add a lot of customers to the PYB fold, but it's likely a lower margin oil being price where it typically is. (B) doesn't really set them apart from other "full synthetic" oils, as they'll surely be dexos1-certified as well. I believe (and yes, it's only a belief), that they went with (C).
(C) Rejuvenate/reinvent Pennzoil's Synthetic Blend oil as an offering dedicated to owners of General Motors vehicles (and there are a lot of them!), and price it somewhere between PYB and PP. They'll look good against other "full synthetic" oils like M1 and VSP because they'll have a lower price, they'll (likely) have a bit more margin with a $19/jug oil than a $16/jug oil, and they'll also reclaim that shelf space that the aging old Gold Bottle was taking up.
So I don't see dexos1 as changing my opinion of a product labeled as a "synthetic blend". I'm sure it's a fine product. But I believe that it's a conscious decision by Shell (in this example) to position their products where they felt that they'd sell best. Could they have gotten the certification on PYB? I believe they could have. And I believe that they chose not to, for the reasons stated above.