From my understanding as read hear at BITOG, this can not be done according to LAW. If an oil company(again, my understanding) makes "X" oil(be it dino, blend, syn or HM), it must be the same no matter which store buys it.
However, a store buying in large qty's like W*M or our favorite large chain auto parts store, then they(the store) can request a particular size/amount in a container. In other words some time ago, JUGS used to be ONLY 4 qts(in the US). Then they moved to 5qts. Now you will see 5+ qts in some instances/some brands/some stores.
Now, for a store/house brand, this may in fact be different. However, the oil must be produced to API minimum specifications and most likely will be produced to a price negotiated by the chain. But it still needs to meet spec.
This is how I understand it.
CB