OOP design problem

I think the accepted method would be to have a ValidFuel(Fuel f) method in your base class that throws some sort of NotImplementedException (different languages have different terms) if the "leaf" cars don't override it.

FillTank could be then be entirely in the base class and call ValidFuel to see if it's valid.

public class BaseCar {
    public bool ValidFuel(Fuel f) {
        throw new Exception("IMPLEMENT THIS FUNCTION!!!");
    }

    public void FillTank(Fuel fuel) {
        if (!this.ValidFuel(fuel))
             throw new Exception("Fuel type is not valid for this car.");
        // do what you'd do to fill the car
    }
}

public class DieselCar:BaseCar {
    public bool ValidFuel(Fuel f) {
        return f is DeiselFuel
    }
}