synthetic does make a difference. High quality products are always a must IMO. You can also request that you get to watch the service being done if you think they will F*** with your Z. I do I never let them take that thing out of my sight!
some dealerships might try to give you some lame excuse like mine did at first but now they don't even bother they know I will be there watching and they are fine with it. All I said is "if you want my business, then I'm watching the service. If this cant be done then I'm not getting anything from you guys." GL and just stand your ground. If it comes down to it you may need to go elsewhere.