What gas fuel to use?

Answer 1

Use the fuel with the octane rating listed in the owner's manual for the car. Consistently using a lower octane fuel can cause knocking, which can damage the engine over time. Using a fuel with a higher octane just wastes your money. Some fuels have ethanol (or other additives) to reduce air pollution, however some of these additives can damage an engine not designed for them. Never use a fuel containing a higher percentage of these additives than is specified in the owner's manual to avoid damage to the engine.
Most modern cars with computer controlled fuel injection (instead of carburetors) can operate on low octane fuel without knocking and the damage that could result, but they do this by the computer making adjustments to the injector and spark timing that sacrifice fuel mileage.

Answer 2

Read the owner's manual.