Yeah, you can argue that they tricked you into believing that you are driving a petrol vehicle when in fact you have a diesel, and vice versa, but that will be hard to prove. My suggestion is to accept that a mistake was made partially on your side (unless you have hard time acknowledging mistakes) and pay penalties/fees and move on...it would be troubling to have a same thing happen again.

Also, i would never have a party travelling with me follow me around to get to the counter to pick-up keys for the rental or room, only when all is settled.