Any oil that Valvoline blends for another company or puts under another name, is not the exact same Valvoline product be it conventional, blend or syn. It's usually blended to a price and within the API spec.
Maybe, maybe not. Valvoline has much more incentive to produce their own branded oil + private-labeled oils in higher volume vs producing their own batch, then a different batch, then a different batch, and so on. Some people will still buy the 'brand' oil even though they make it for others - there's always that "maybe it's not exactly the same" thinking.
You are correct however, will we ever know?
Also, I was only stating what a Valvoline Rep told me over the phone be it wrong or right! If you ask 5 different REP's, you'll get 5 (slightly) different answers.
I use lots of store brand oils as they(according to the PQIA) seem to be just fine for everyday use, as they should be. However price being the same, I'll always use the name brand for sure...why not?