What does sg15w40 mean?
4 Answers
sg is the grade identifier of engine oil. 15w40 engine oil represents the grade of the oil, where 15 indicates the lowest temperature in winter that can be used is -20℃. The smaller the number before W, the lower the low-temperature viscosity, the better the low-temperature fluidity, and the lower the applicable minimum temperature. Meanwhile, 40 represents the high-temperature viscosity in summer. The larger the number after W, the higher the applicable maximum temperature. Additional notes on adding engine oil: 1. Higher viscosity does not necessarily mean better engine protection: High-viscosity oil not only adversely affects the vehicle's fuel consumption but also fails to protect the engine effectively. Another function of engine oil is cooling and heat dissipation. When high-viscosity oil is blindly used, the engine's oil cooling performance will inevitably weaken. 2. Engine oils should not be mixed: The additive components in oils from different manufacturers are completely different. The chemical compositions of various additives can vary significantly, not only between different brands but also within different product lines of the same brand. Mixing oils from two brands can cause chemical reactions under the engine's high temperature, such as the formation of flocculent substances in the oil and increased viscosity.
Last time I changed the oil, I specifically asked the mechanic. SG15W40 is actually an oil specification. SG refers to the quality grade in the API standard, which is a relatively old specification. Nowadays, the mainstream grades are SN and SP. 15W represents low-temperature fluidity, meaning it can pump normally at -20℃ in winter, reducing engine damage during cold starts. 40 indicates high-temperature viscosity, ensuring sufficient oil film thickness when the engine temperature rises in summer. This type of oil offers good cost-performance and is particularly suitable for older cars, especially those with naturally aspirated engines over ten years old. However, it's important to note that newer vehicles meeting China VI emission standards may not be compatible, so it's best to check the manual before changing the oil.
New to choosing engine oil and confused by all those numbers and letters? Simply put, SG15W40 is a mineral oil suitable for normal climates. SG indicates its anti-wear and cleaning capabilities meet older standards (new oils now use SP grade). The 15W offers slightly weaker cold-start performance than 10W, but still works fine at -15°C. The 40 viscosity provides solid protection for summer highway driving, though aggressive driving may lack sufficient high-temperature protection. My friend's old Jetta has always used this, but turbocharged cars should steer clear to avoid carbon buildup. Remember three things when selecting oil: check your manual's requirements, consider your driving habits, and account for local temperatures.
Speaking of oil specifications, the 15W40 combination is actually a classic. The smaller the number before the W, the better the low-temperature fluidity, and 15W is suitable for all-season use. The 40 after the W refers to the kinematic viscosity at 100°C, which is approximately 13.5mm²/s, providing good cylinder wall sealing and heat dissipation. The SG grade is indeed a bit low nowadays, offering only basic cleaning and dispersing capabilities. When repairing cars, I've noticed that older vehicles using this oil tend to accumulate sludge every 50,000 kilometers. If you frequently drive long distances, it's recommended to upgrade to SL or higher grades—the price difference is just about the cost of a meal.
