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Whether it is Shell Helix or other brands of engine oil, there are many English and numbers on the oil barrel, which is often confusing to novices. Despite the repairman's recommendation, the average car owner still doesn't know the answer. In fact, as long as you understand simple rules, you can understand the logo on the oil barrel and easily choose the right lubricant for your car. SAE is the English abbreviation of Society of Automotive Engineers, and SAE grade represents the viscosity grade of oil. SAE30 and SAE40 are single-grade oils, while SAE10W-30 and SAE15W-40 are multi-grade oils. Among them, the smaller the number in front of "W", the lower the low-temperature viscosity, and the better the protection ability of the engine during cold start. ； “The number after "W" is an indicator of the high temperature resistance of the engine oil. Taking Shell Helix Super Motor Oil as an example, the most typical data is SAE10W-40. API is the abbreviation of the American Petroleum Institute, and the API grade represents the classification of engine oil quality. It uses simple codes to describe the engine oil's ability to do its job. API engine oils are divided into two categories: “S” series represents gasoline engine oil ； “C” series represents diesel engine oil ； When the letters "S" and "C" exist at the same time, it means that the engine oil is universal for gasoline and diesel engines. If "S" comes first, it is mainly used for gasoline engines. On the contrary, it is mainly used in diesel engines. With each letter from "SA" all the way to "SJ," the oil performs better than the previous one and contains more additives to protect the engine. The further back the letters are, the higher the quality level. Among international brands, most engine oil grades are SF or above. For example, Shell Helix Plus (Shell Helix Plus) is APISJ grade, while Shell Helix Red Motor Oil (Shell Helix Red Motor Oil) is APISG grade, which shows that the quality level of Extra Helix is higher than that of Red Helix. Since the same type of engine oil has different temperature characteristics in different engines, the selection of engine oil should still be based on the model recommended in the manual! If there are different recommended oil types in the manual, you should find out your car's engine and the specific temperature characteristics of these recommended oils. Common engine oil models: 5W-30 10W-30 10W-40 15W-40 20W-50 W" means multi-grade engine oil, which is an engine oil with a wide applicable temperature range. It is added with high molecular polymer substances. In addition to the characteristics of general engine oil, its viscosity can be adjusted within a certain range, thickening when it is heated and thinning when it is cold, which can reduce the resistance and wear of the engine. Take 10W-30 as an example: "The first 10W represents the low temperature viscosity index. Because it is a low temperature index, at low temperatures, this value indicates the difficulty of turning the crankshaft and starting the engine and the lubrication performance of the oil flowing to all parts of the engine. The smaller the value, the lower the low-temperature viscosity, which is better when starting at low temperature. The following 30 represents the high temperature viscosity index. Provides and guarantees concentration, viscosity and good lubrication properties under operating high temperatures. The larger the value, the better the high temperature performance. 10W-30 and 5W-30 are now the two most widely used engine oils, because these two engine oils basically cover the normal temperature range. They have both the "thinness" of starting at low temperatures and the "concentration" guarantee at high temperatures. Both 10W-30 and 10W-40 are suitable for temperatures above -18 degrees Celsius, but at temperatures below minus 5-10 degrees Celsius, 10W-40 does feel significantly "thicker" than 10W-30 (although the low-temperature parameter of 10W is the same). If the temperature is lower than -18 degrees Celsius, it is recommended to use 5W-30 ； However, 5W-30 has a high temperature limit and is suitable for use below 40 degrees Celsius. ； And in some engines, the maximum applicable temperature of 5W-30 will be reduced to about 15 degrees Celsius. The operating temperature range of 20W-50 is above freezing. Therefore, it can be seen that the lower limit of low temperature of 15W-40 should be between -18 degrees and zero degrees (unknown). 10W-40 is more suitable for summer oil. This is actually based on experience. In cities with temperate climates, 10W-40 is usually used in summer and 10W-30 in winter. Of course, depending on the engine, some use 10W-30 or 5W-30 all year round. If you look at the size of the engine, the recommended oil for small engines (cc rating=1.8L) is generally "thicker". The correct selection of engine oil, especially in winter, is very important. The viscosity of engine oil changes with temperature. The low temperature in winter makes the viscosity of engine oil increase and thicken when it is stationary at night. This is also the main reason why the oil pump sometimes has difficulty pumping oil and is difficult to start when starting.