All gasoline sold in the United States is required to contain effective detergent additives. Use of additional detergents or other additives is not needed under normal conditions and would result in additional cost. Therefore you should not have to add anything to the fuel.
See also:
TIREFIT Kit
Small punctures up to 1/4 in (6 mm) in the tire tread can be sealed with TIREFIT.
Foreign objects (e.g., screws or nails) should not be removed from the tire. TIREFIT
can be used in outside temper ...
Steering
...
Snow tyres
Use snow tyres of the same size as the normal tyres provided with the car.
Fiat Dealership will be happy to provide advice concerning the most suitable
type of tyre for the customer’s requirements. ...