Ethanol has a higher octane than gasoline. It has a lower energy density but that would not factor into this discussion.

10% Ethanol should not be aggressive (corrosive if you are not Politically Correct) enough to attack any non-modern fuel components (natural rubber and less chemical resistant plastics). This should not be a problem with a 240 anyway.

The two things make a flex-fuel car capable of running on Ethanol (E85 and theoretically E100) without major side effects. First, the above mentioned fuel system components. Second is a fuel and ignition computer capable of the wide range of adjustment required to handle a variety of fuels...a wide-band O2 sensor is almost always required.

I typed that last paragraph to say this: something on your car is "marginal" when running on regular fuel and becomes outside of the fuel-injection or ignition maps when running Ethanol. Do a tune up with high quality components to include replace the ignition wires and cleaning the throttle body and idle motor.

Mike