The label on the fuel cap is the Minimum Requirement for your car to run.

Its like you just taking water instead of other drinks to survive.

Your engine's computer will give more ignition/ spark advance on higher octane fuel, resulting in better response & more kms per liter.

Try running different grades of Petrol to compare the way the engine responds & what kms per liter you get.

Based on the results you get, stick to the Octane Grade that gives you the best kms per liter.

Don't forget to check your Tire Pressures as well.