Have you looked at AZ shelves. All the AZs I have visited, and for that matter AutoShacks before them, could not physically load oil from the back of the shelf. There is no room back there. So unless they are willing to pull all oil off the rack to restock and then reload the old oil, then the last bottle is the oldest. I would wager that for many oils that have never been sold-out completely, that the last bottle is the original bottle that was stocked before opening day of the store. The only thing that gets that last bottle off the self is when they re-organize the shelf arrangment. On that day, I would wager, the last bottle may actually get rotated to the front of a new shelf position. Sometimes...by mistake...maybe...