I agree with a bit of what Harvey is saying, but don't think ZV is totally wrong, either.
First, yes...the dealership should have offered to replace the oil with synthetic. IF that is what the customer asked for, and IF the customer actually pointed this out while paying the bill and didn't just come home and gripe on the internet about it.
As far as whether the dealership "should have known" the customer always used synthetic.....I don't see any clear path here. It's up to the customer to remember what they want and see that they get it.
If I order "the usual" at my favorite restaurant and they make me something else, it's my fault if I don't point it out when I'm served and then specifically tell them what I want.
That's a bit different, but not a lot.
It's not going to matter, because there isn't going to be a problem.....you'd play hell proving that using dino oil instead of synthetic caused any problem, and then and only then would the issue of "did customer order synthetic" and "was customer charged for synthetic and got dino oil" come into play.
Also, you'll have other arguments, such as the dealership possibly saying "cust asked for an oil change, got an oil change, didn't specifically request synthetic" and you're down to "he said, she said".
Definitely arguing semantics on this one.
When I did that job, I did not check to see what type of oil a car got the last time it was there. Customer asks for an oil change, they got an oil change. If they requested something in addition, they got that, too. But checking to see what type of oil someone got last time is not standard procedure, no matter if car geeks think it should be. I can easily see someone making that mistake.
So the proving of the "dealer should have known that's what customer always ordered" would not be cut and dry.
Best to just call it a minor mistake and move on.