40-grade motor oil has a higher viscosity than 30-grade motor oil. The larger the number, the higher the viscosity of the oil and the higher the maximum applicable temperature. The SAE (Society of Automotive Engineers) viscosity rating refers to the standards set by the Society of Automotive Engineers in the United States, which are universally adopted worldwide. Oil viscosity is the most important characteristic of motor oil, simply put, it is the resistance of a fluid when flowing. Generally, terms like 'thick' or 'thin' refer to the size of the viscosity. Neither too high nor too low viscosity is good for engine oil; 'the right viscosity' is the best. This is because both excessively high or low viscosity have the following disadvantages: 1. If the viscosity is too high, the oil is thicker and cannot be quickly delivered between parts in a short time, which increases wear during startup, makes cold starts difficult, reduces cleaning and cooling effects, and leads to greater power loss and increased fuel consumption. 2. If the viscosity is too low, the oil is thinner and cannot effectively adhere between parts, resulting in poor lubrication, anti-wear, and sealing effects, and also increases oil consumption. 3. In theory, as long as the oil does not deteriorate and is of good quality, its oil film will persist, fully utilizing the oil's performance to protect your vehicle. 4. However, if the oil contains too much water, its lubricity will deteriorate, viscosity will decrease, and the anti-friction effect of the oil film will weaken, inevitably leading to oil deterioration and potentially more serious accidents, such as engine seizure or bearing failure.
