Besides its convenient battery-powered string trimmers, Ryobi manufactures gas-powered ones for more heavy-duty applications. Some have two-stroke engines that run on a mixture of gas and two-stroke engine oil, and others have four-stroke engines to which you add oil independently of fuel. If yours is a two-stroke model, you won't find an oil crankcase, and you won't see an oil cap. If you aren't sure about your model, consult the manual.
Ryobi recommends a 50:1 mixing ratio of gasoline to oil for its two-cycle string trimmers. That translates to 2.6 ounces (76 milliliters) per gallon (3.8 liters) of fuel. You usually don't have to measure this quantity of oil yourself because two-cycle oil comes in premeasured bottles. To mix the oil and gas:
Things You'll Need
1 gallon of gasoline
Open the oil container, note the level of the oil and pour oil into an approved container. If you're mixing 1 gallon of fuel, pour until the oil level goes down to the next marker, which means you just poured out 2.6 ounces. Virtually all two-cycle oil containers have the same standard markings because nearly all contemporary two-cycle engines take a 50:1 gas/oil mixture. Some bottles are small enough to contain a single 2.6-ounce portion of oil. If you buy a bottle of two-cycle oil without level markings, you'll also need a measuring cup to measure the oil volume.
Shake the gas can to thoroughly mix the oil with the fuel.
You can mix more than 1 gallon of fuel at a time, but mix it in increments of 1 gallon if you want to take advantage of the markings on the oil container. Otherwise, measure the oil in a measuring cup.
Adding Fuel to a Four-Cycle Trimmer
The most heavy-duty of Ryobi's trimmers come with four-cycle engines, and if you have one of these, you'll see the oil cap on the back of the engine housing, under the muffler. Check the oil level by removing the oil cap, cleaning the dipstick, screwing he cap back on, then removing it again. The trimmer should be resting on a level surface while you do this. Add oil if the level is below the bottom mark. To avoid overfilling the crankcase, add oil incrementally, checking the level until it's flush with the top mark on the dipstick.
Two-Cycle and Four-Cycle Oil
You don't use the same oil for two-cycle and four-cycle engines. Because a two-cycle engine burns oil along with the fuel, it's formulated differently than four-cycle oil, which gets constantly recycled. Two-cycle oil is clearly labeled as such, and there are two grades -- one for land use and one for marine use. Be sure to choose a product intended for land use. If you have a four-cycle trimmer, the recommended oil is Ryobi four-cycle 20W50 engine lubricant. You can also use SAE 30, 10W30 or 10W40 engine oil -- in short, virtually any oil suitable for an automobile engine.