What octane of gasoline should I use in my engine?
Always use unleaded automotive gas with a minimum octane rating of 87 AKI (91 RON). If operating at high altitude, the same type of fuel can be used but an additional carburetor jet kit might be required. Do not mix oil with gasoline. (Pages 13 and 26)
From the official Briggs And Stratton 500 Series manual, page 13
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