If your weed trimmer has a two-stroke engine, it needs two-cycle engine oil in the gas to operate correctly. Since two-stroke engines don't have a dedicated lubrication system, oil is mixed with the gas to keep the crankshaft lubricated. The proper ratio of this mixture is essential to keep the weed trimmer operating efficiently. Too much oil can cause the engine to gum up; too little oil won't lubricate it sufficiently. It is, however, easy to determine the oil-to-gas ratio in a weed trimmer.
Inspect your weed trimmer in a well-lit area. Near the gas tank of most weed trimmers, either imprinted on the gas tank cap or on the gas tank itself, you will see a set of numbers like 50:1 or 25:1. This is the ratio of gasoline to oil.
Look in the manual for the weed trimmer if you cannot find the ratio printed on the gas tank. The manual will give you the exact mixing ratio for your particular brand and model of weed trimmer.
Calculate the amount of two-cycle engine oil that should be added to your gas. For example, if your weed trimmer has a ratio of 50:1, you would need about 2.5 oz. of oil for 1 gallon of gasoline. There are 128 oz. in 1 gallon, and 128 divided by 50 is close to 2.5.