Your Subaru needs good quality, clean fuel. You wouldn't drink swamp water, so don't expect your Subaru to be very happy if you let it drink the wrong fuel!
The good rule of thumb if your Subaru doesn't require high octane fuel is to buy the best fuel you can afford. We have many customers who have told us they use premium 95 or 98 octane fuel in their vehicles that only require 91 octane because they feel the car drives better and some have mentioned getting better fuel economy. Performance Subaru models like WRX, Levorg and BRZ require a high octane fuel to run efficiently.
The most important reference is your owners manual, but Sam from Subaru Australia made this informative video for us some time ago to help explain it.