I suppose you have to trust that they do, but how do you know what they put in in any case. That's possibly why its best to supply the same as they put in; that way they'd have nothing to gain. I know on one occasion they forgot to put in the Magnatec (which was what they were putting in at the time) that my father had provided for them, and he came away with two five-litre cans and no charge for the oil despite the Service Manager being aware.
If you supply the same oil as they do, then I cannot see how they can refuse to use it. If they did, I think I'd take my custom elsewhere to a Stealer who would. The only ones I have heard of being difficult are VW, because they want their grossly overpriced superiorly engineered and all that rubbish stuff to be put in. However, if the oil you provide is to that Manufacturer's specification, how can they legitimately refuse?