I worked at a tire/ parts store and we did bottled oil changes at a reasonable price, using oil from our own store, obviously. We had a huge window so the customers could watch us like we were in a fish bowl and the system worked fine.
We had an idiot show up with a mitsubishi with the dodge neon 2.0 motor, and a wix filter with 20mm threads intended for the other mitsu motor that he got from NAPA because he "knows better". We put a PH16 with 3/4 threads on instead and he shrugged it off. 20mm is just big enough to thread/jam on then blow off later, so I'm glad my BITOG nerdery kicked in that day.