It is generally recommended to use the fuel specified by the motorcycle manufacturer in order to ensure optimal performance and longevity of the engine. In the case of the Royal Enfield Classic 350, the manufacturer recommends using unleaded gasoline with a minimum octane rating of 91.
Mixing additives or other fuel products, such as Mobil, directly into the fuel tank is not generally necessary or recommended. The fuel sold at reputable gas stations already contains additives to enhance performance and keep the engine clean.
If you have specific concerns or requirements for your motorcycle's fuel, it is best to consult the owner's manual or contact a Royal Enfield dealership to seek their advice. They will have the most accurate information and can provide guidance based on your specific needs.