FlyerTalk Forums - Sixt: Wrong Fuel

I rented a car from Sixt back in August. I used rentalcar.com so I didn't know which company I would get until I confirmed the reservation. I picked up a car at Schipol Airport(took me 2 hours! Sixt was the only company that had such a long queue, I wonder why?) and was supposed to return it at Frankfurt Airport. I got a Ford Focus with automatic transmission. On the last day I had to drive from Venlo, Netherlands to Frankfurt airport but the screen showed up with "Engine Malfunction" after driving for about 2 hours already. I stopped a car at the parking along the highway and couldn't start the car again. Called Sixt and they said they would send someone within an hour. An hour and a half passed nobody showed so I called again and this time I also went to SOS box as well. About 15 minutes later someone from SOS came and magically the car started just fine and there was no "Engine Malfunction" coming up again. Sixt wouldn't have sent anyone because they said they would call me once they were near. I could drive for another 30 minutes and the car stopped. A police came and took Sixt number to call for me. He came back and said that Sixt said they wouldn't send anyone so he called a tow truck for me. That tow truck dropped me off at Ford center and told me to take a cab to the airport and to show them this piece of paper he gave me. Once I got to the airport, I gave them this paper and he said OK everything has been taken care off. I got a mail letter from Sixt on Friday saying that I needed to pay 881.28EUR by October 2 because I put the wrong fuel in. I just noticed from their documents that their Ford Focus needed Diesel. I then realized I was wrong, I put Benzine in. I know it was my false but do you guys think I can negotiate the price a bit? I mean none of their staff told me it needed Diesel, no sticker on the car or anything. Ford Focus here in Thailand needs Benzine so there's no way I could have guessed it would need something different there. Thanks!