I'm not sure what dealers you've been doing it with. But for me whenever there's a major issue you don't deal with the dealer, you deal directly with corporate, and they want ever bit of service record you have. That's how they find out exactly what oil you've been using.
With super high end companies like bmw, they even scan the dme for tunes, abuse (over revs etc), and second an engineer out for inspection before they warranty anything. So final rulings are directly from corporate, if you disagree the next step is litigation. You're not dealing with a dealership for big issues...