There is a lead additive available at auto parts stores. Lead provides a critical wear reducing function by depositing a thin, protective layer of lead oxides on valves and valve seat surfaces reducing the risk of Valve Seat Recession. Without a Lead Replacement Additive, the valves will burn...
Changing a worn O2 sensor will increase mileage. Follow the exhaust manifold around and find the O2 sensor close to the bottom. Buy the one wire OEM, not the universal.
At idle or really low speeds, the gas engine turns off and the electric motor comes on. When you take off or increase your speed, the electric motor turns the gas engine back on. If the battery charge is low, the gas engine will remain on.