There are large generic drug firms which can completely duplicate drugs whose patents have expired. If it says compare to Nyquil, in general the ingredients are the same. If the patents hadn't expired there would be an infringement lawsuit. There are a few generic blending plants for motor oil. Specialty Petroleum, now a Quaker State (Pz,Shell) subsidiary was once independent.
For the most part, however, there are a limited number of refineries. I would think "compare to Havoline"--with the copyright being placed with Chevron, not Equilon--means that Chevron has some extra plant capacity that's being resold by Advance.
We all know that gasoline is largely a commodity and that most gasoline in a particular area will be from a very limited number of sources despite the different brands on the pumps. Bottled oil would have a different distribution channel, but I'm not totally convinced that something similar might not occur if it's to the oil company's advantage to do so. This would explain the varying Shell formulations that CR found. This would be very easy to do with bulk oil.
Here's a bit of speculation for those of you who like to buy name brand oils at Wal-Mart. Wal-Mart has a house brand tire Douglas which is made by Goodyear (Kelly). They also have a branded tire, BFGoodrich Excentia, which is exclusive to Wal-Mart. Wal-Mart has such large buying clout that is it possible that they control specs. on some branded oils? Others have reported that Delco oil filters at K-Mart (no longer carried) and Wal-Mart were different products, especially for non-GM applications. Could the same apply for oils? Probably not, but it's not completely impossible.