According to the Lawn-Boy 320E operator manual, the fuel mixture ratio is 40:1. This means that there should be 40 ounces of unleaded gasoline to every 1 ounce of 2-cycle engine oil. One gallon (US) is equal to 128 ounces, so you would need 4 oz (1/2 of a cup) of oil to 1 gallon of gas.
