This right here. Nothing wrong with buying in bulk, as long as you have receipts and document each maintenance interval. I keep a log in the glove box of each vehicle, along with an envelope for receipts. If it's a bulk order, for all my vehicles, I run a copy for each car, plus keep an electronic copy. Oil filter box gets the part number/UPC torn off and put with the receipt with date of install written on it.I just keep the store receipts from my oil and filter purchases and keep a paper log where I write down the date, mileage, and what services were performed. I put those in a folder with any other receipts for things like tire and battery purchases. I've yet to ever be asked to present this documentation to get a warranty repair completed.
Really should't be a problem. Pretty easy to figure out whether someone has been changing the oil or not. If you break out the documentation, it's on them to prove it was something you did or used that caused the failure.