Unified SG 15W-40, is this not as good as the special engine oil for Great Wall?

API gravity (API) standard is usually adopted as the grade index of engine oil in the world. API is usually represented by two English letters, the first English letter is the appropriate engine model, S stands for gasoline engine oil, and C stands for diesel engine oil; The second English letter stands for petroleum grade, arranged alphabetically. The higher the arrangement of the second letter, the higher the quality, which can provide better protection for the engine. For example, the performance of API SL is higher than that of API SJ, SH and SG oils. Usually, high-performance vehicles use API SL or SJ engine oil; Generally, domestic vehicles use API SG or SF grade engine oil. Oil can also be selected according to the year of manufacture of the engine. SF is suitable for gasoline engines in the mid-1970s or late 1980s. SG is suitable for gasoline engines in the late 1980s. SH is suitable for engines produced after 1993 and before 1998; SJ is suitable for engines produced after 1998. Viscosity and temperature index we only know the classification of lubricating oil, which is far from enough. We must also choose the oil by viscosity and temperature. Viscosity index: internationally recognized classification is SAE classification, which is the British abbreviation of American Society of Automotive Engineers. In SAE classification, it is divided into low-temperature viscosity grades SAE 0w (w stands for winter), 5w, 10w, 15w, 20w and 25w. The smaller the number, the better the low temperature operation. For example, 0w represents good lubricity at minus 30 degrees. 5w stands for minus 25 degrees; 15w stands for minus 15 degrees; 20w stands for minus 10 degree; 25w is 5 degrees below zero. In addition to low temperature viscosity grades, there are also high temperature viscosity grades SAE 20, 30, 40, 50 and 60. The higher the number, the higher the viscosity of the engine oil. Usually, oil with high viscosity is used in hot weather and oil with good fluidity and low viscosity is used in cold weather. In addition, high-end cars or new cars use oil with good fluidity and slightly lower viscosity because of the accurate clearance between parts, while old cars can choose high-viscosity oil with relatively condensed sealing performance because of the large wear between parts. SJ grade is higher than SG grade. It is better to use a lower viscosity in winter, 30 is stronger than 40. I hope it helps your car.