Don't confuse octane with the quality, or power of fuel Octane is just a rating of how fast the fuel burns. If you have a high performance car with an engine designed for high octane then it needs high octane gas to provide maximum power and in some situations it needs it to provide max mileage, but that is not common. High octane gas is just a little slower burning.
You don't need to know all that. All you need do is to find out what fuel the manufacturer specifies or recommends. If it is specified then you must use it or risk engine damage. If it is recommended, then not using it will result in less power and maybe less mileage.