Sorry for the expensive lesson, but you are responsible. We were very careful when we first rented a car in Paris, many years ago, to ask the gas station attendant which of the Diesel/Petrol types to use. SIXT was our rental company as well and said to use diesel. When we got to the station and saw three different diesels, we asked the attendant for help. He kindly pointed out the right selection and we never had a problem after that. Clearly the SIXT agent thought you knew that there was a difference in the types of Diesel fuel for cars in Europe, but as a consumer you bear responsibility for making sure you are using the proper product when you get to a fueling station. If it was unclear to you, then you should have asked the attendant for help, before you put the wrong fuel in the car. Expensive life lesson, and maybe SIXT will cut you some slack, but they are not responsible for your petrol selection at the gas station. Asking questions is not a sign of weakness, nor is asking for directions.