All leaded and unleaded gasoline sold in this State shall meet or exceed the following minimum octane numbers:
Regular Grade 87
Midgrade or Plus 89
Premium or Super Grade 91
An octane number is determined by adding the research octane number to the motor octane number and dividing by 2. (RON + MON)/2. In addition, the motor octane number shall not be less than 82.0. All gasoline products sold at retail shall have an octane number displayed.
815 ILCS 370/4