Me? Well, of course, if it went to court, we know that all bets are off and everyone might try to get themselves excluded from a suit and/or add everyone else in sight to it. Aside from that, if I found that another identical jug (same product, same batch number) had this sketchy product in it (i.e. by a VOA), then I'd proceed to contact the oil company and seek information or redress from them. It's possible they could verify it as a counterfeit product (for example, by an imaginary lot number, label errors, or other product markings), or it could be confirmed as their own. If it were counterfeit, I'd be after the retailer, who I'm sure would be interested in what was going on and what they were getting and from whom.
Now, if it weren't a counterfeit product, in that the VOAs didn't match (one seemed like ordinary motor oil and the other in an identical jug was the mess we've come across), I'd gather that someone took great pains to buy Mobil 1, return something in its place for a refund, and foist an inappropriate product on an unsuspecting consumer. I'd be after the retailer again. Now, if someone did the oil change for me, that adds a little more complexity, and more suspects.
I'm not trying to give any oil company a free pass here, but it just seems to me, and we've had formulators discuss the procedures followed when filling and shipping product before, that the odds of XOM filling an M1 jug with something that is decidedly not motor oil are pretty slim, and when it does happen, it's a much wider problem with an entire batch. If XOM were filling M1 bottles with a product that were blowing engines, we, of all people, would be hearing about it, even before XOM customer service. That's not to say that cannot happen, or even didn't happen here. The first place I'd be looking, however, is that someone did some pretty fancy tampering (it's happened before) or that a counterfeit product hit the shelf.
As I mentioned, though, this is all cold comfort to the victim in this situation, since he's the one stuck trying to track down what went wrong, not to mention dealing with the inconvenience and the cost. Additionally, all this inconvenience and cost still exist for him, regardless of whom is at fault. If XOM goofed, it would be nice of them to correct the situation on an individual basis, and to ensure that there isn't some procedural problem. If a retailer goofed, aside from making things right, some of these return procedures should be reviewed.
On a wider point, I'm generally not a fan of the notion of returning products unless said product is defective or the retailer/shipper provided you with the wrong product. If you don't know what you're buying (as in oil grade or filter part number), stay out of the store until you know what you're buying. So many places just take things back without questions, simply to avoid fights and arguments, and people take advantage of that, and the next purchaser winds up suffering. Other posters here have seen used oil on the shelf. Here, I know people from electronics stores that have people buy a big screen TV on the Friday before Super Bowl, and then return the stupid thing on Monday. Gee, that's not suspicious at all.