A Black Max Weed Eater has a 2 cycle engine meaning it needs a mix of gasoline and oil in order to properly operate. Do not try to operate your Black Max with just gasoline in the fuel tank.
You should purchase synthetic 2 stroke engine oil and mix it according to the directions on the bottle you've purchased. This string trimmer needs 2.6oz of oil per gallon of gas.
Black Max sells synthetic oil for 2 stroke engines and suggests that their 2.6oz bottle should be mixed with a gallon of regular gasoline. Black Max recommends not adding old fuel to the fuel tank.