If you have to ask, you need to find another dealer, or do it yourself. And some customers are cynical or paranoid by nature, hopefully not you. Oil changes are simple and profitable. I think it would be one of the last places a dealer would cheat. With that said, the oil changers are not usually long-term, master mechanics, and certainly one could cheat on the invoice and sell the oil and filter out the back door. I try to develop a good relationship with the shop foreman or a senior tech, and it seems to pay off in good service.
2011 Space Gray Coupe, DCT, ZCP, StopTech ST-60/ST-40 BBK.