Just my opinion from my dealer days:
Yes, sometimes billing mistakes happen. They may really be pushing the synthetic, but did accomodate your request for dino. Between what the tech wrote down (which may or may not have been correct) and what got processed on the repair order up in whatever constitutes the billing office (which may mave either reflected a mistake in the shop, or which may have been screwed up by the billing office), mistakes can and will on occasion happen, it's a fact of life, I guess. It may have even gotten synth put in it, sure. You're dealing with human beings here. I'm sure no one intentionally put oil in the car you didn't want. Hey, at least they DID put oil in it, look at some of the screwups you hear about (no oil put in, forgot the drainplug, forgot to tighten the filter, etc etc.
Not to belittle your concern, it's upsetting when you don't know who to believe. In my opinion, I'd just go along with the belief that everything is how you want it. If you don't want to believe it, it sets up really bad feelings towards the repair shop. If you now distrust the shop, then find another one to go to and get the oil change done there. Be sure to explain to them WHY you are there, that oughtta put them right at ease
. Either that or TRUST that they did it the way you want and keep going back there. Maybe have the oil change done slightly before the normal interval if you are still uneasy about the "last oil change" thing. And you can always try doing something to help ensure you're getting the proper oil the next time, maybe write "dinosaur guts only" on the valve cover. BTW this whole distrust of synth oil is a little far out there for me. It all comes out of the ground you know. Nothing is truly "man made" except other little people, and I think a woman is involved in that, too. Manifesto over.
Gilly