Your Subaru is designed to operate on unleaded fuel. The minimum octane rating of the fuels used must be at least 91 RON. All unleaded fuels sold in the U.S. are required to meet this minimum rating.
Use of a fuel too low in octane (a measurement of anti-knock quality) will result in spark knock. Since many factors affect operating efficiency, such as altitude, terrain, air temperature and humidity, knocking may result even though the recommended fuel is being used. If persistent knocking occurs, it may be necessary to switch to a slightly higher grade of gasoline. Continuous or heavy knocking may result in engine damage.