I often find on my MG that if they do have the part, they spec something different in their system than what the factory said or different from what I prefer to use.
A good one is on spark plugs. I still use(thanks to a big stockpile) the now-discontinued NGK BP6ES plug. The book says Champion N9Y(or N9Y-c by a newer number) but their system says NGK BPR6ES.
I've gone in before and asked for spark plugs by the NGK part number, either BP6ES or the stock number, 7333. I often get a deer in the headlights look when asking for that, and they want Y/M/M rather than even look it up. I've done the same thing with a certain model Bosch alternator that, again, is a drop in but not listed in their system. Often times it takes a manager to get something by part number rather than Y/M/M. I've even been asked for that before when I wanted to buy a timing light(pointing to one behind the counter on a rack) and they wanted to "make sure it fit" my car.
BTW, all of this happens at the the big three "consumer oriented" Autozone/Advance/O'Reilly's. I find NAPA generally has pretty good employees, and the one Carquest store tucked in the corner of an industrial park is AWESOME.
Around here, O'Reilly's will turn disks and drums. All the MG drums I've seen have "10.06 in. MAX" cast rather boldly into the inside of them. I took them to one store and they absolutely refused to touch them because the specs weren't in their system. Apparently their computer knows more than the place that made the drum. At another location, the manger was at least willing to try, although with them in hand he wouldn't because he measured them at 10.05. I should say that he was still WILLING to turn them, but said that I thought a .01 cut wouldn't accomplish anything on them and he thought I'd be wasting my money doing so. I at least appreciated him being willing to look at the drum and actually give me an intelligent answer on turning them, even though he didn't end up doing it.