Ryobi makes three types of trimmers. One of them is electrically powered, which means it has no fuel source. The other two types are gasoline powered trimmers that require either a 2-cycle gasoline and oil mixture, or a 4-cycle engine that takes regular pump gasoline. The correct fuel replacement is critical for each type of gasoline powered trimmer, and incorrect fuel replacement could lead to a catastrophic failure of the engine.
Determine what type of Ryobi gasoline powered trimmer you have. The 4-cycle version will be clearly marked on the engine compartment in bold and easy to see letters and numbers. The 2-cycle, which is the more common of the trimmers, will generally have an embossed marking saying "2-cycle" on the gasoline cap.
Pour pure gasoline, straight from the pump or out of your gas can, into your 4-cycle engine's fuel tank. All gas cans come with a funnel extension which will fit inside of the tank and allow easy pouring with little or no dripping.
Mix gasoline for your 2-cycle trimmer with 2-cycle oil before you fill your gas tank. Ryobi 2-cycle trimmers take a 32-to-1 gas to oil mix. Add 4 oz. of oil to 1 gallon of gasoline, then agitate the gas and mix them together for about 60 seconds. Mixing oil with the gas is crucial because 2-cycle engines get all of their lubrication from the oil/fuel mixture. Leave the oil out, and your engine will burn out within minutes.