Do you have the original promise/agreement in writing ? Not just paraphrasing what they told you, i.e. he said, she said.
I will see if I can find it. It was just always $20 oil changes for the life of the car. I never thought about it until I looked at the price of synthetic oils, the price I was paying, and the way they call it a blend and came to the conclusion there was no way I was gettting a standard synethitc oil sinice they didn't even call it that.