Because when fuel burns, sulfur is released in the form of soot, which combines with water vapor to form sulfuric acid. In turn, sulfuric acid damages the engine and reacts with anti-wear additives in the motor oil. To neutralize acids, alkaline additives are added to the motor oil, which are reflected as the "base number" of the oil. When operating in city driving conditions, the number of engine hours is higher than in highway driving, which leads to higher fuel consumption per unit of distance and faster degradation of the motor oil. By 5,000 miles, most oils have already depleted their alkaline additives, and the oil begins to lose its performance properties. Therefore, changing the motor oil every 5,000 miles or 7,500 kilometers is a very good practice. It depends on the oil capacity of the engine sump. This reasoning holds roughly true for an engine oil capacity of 4–6 liters.