What is the difference between fully synthetic 5w30 and 5w40?

[KalebMarie]

The difference between fully synthetic engine oils 5w30 and 5w40 is: The smaller the number before W, the lower its low-temperature viscosity, the better its low-temperature fluidity, and the lower the applicable minimum temperature. The larger the number after W indicates the higher the viscosity of the engine oil. The number after W mainly refers to the indicator of the oil's high-temperature resistance; the larger the number, the better the protective performance of the oil at high temperatures. Here are methods for choosing engine oil: 1. Based on vehicle age: Generally, referring to the vehicle maintenance manual, new cars should choose oil with slightly lower viscosity, where 5W30 would be more suitable. As the vehicle ages, choose oil with slightly higher viscosity. Older car engines experience more severe wear, thus requiring relatively higher viscosity oil for smoother and more comfortable driving, where 5W40 would be more appropriate. 2. Based on road conditions: Road conditions can greatly affect the lifespan of engine oil. In areas with poor road conditions, the oil change interval should be shortened. In urban areas with frequent traffic congestion leading to frequent stops while driving, it is recommended to use oil with lower viscosity. If you frequently drive long distances or on highways, slightly higher viscosity is recommended. 3. Based on temperature: Due to significant temperature differences between summer and winter, oil pressure gauges generally read lower in summer. However, as long as it is within the reasonable range specified in the manufacturer's manual, it is normal, and car owners should not be overly concerned. If the oil pressure gauge is unusually high or low, deviates too much from the normal situation, or is outside the reasonable range, then a thorough inspection is needed to check if there are any issues with the engine, such as a faulty sensor, clogged oil passage, etc.

[JulianRose]

Having driven for many years, I often struggle with choosing between 5W30 and 5W40 fully synthetic engine oils. 5W30 has lower viscosity, meaning it flows more smoothly during cold starts, improves fuel efficiency in city driving (saving approximately 0.5 liters per 100 km), and is especially suitable for winter or gentle driving habits. However, in high-temperature environments, such as summer highway trips, its protective properties are slightly weaker, as the oil film may not be thick enough, posing a marginally higher risk of engine wear. In contrast, 5W40 offers stronger viscosity at high temperatures, providing more robust engine protection, making it ideal for aggressive driving or older vehicles to reduce component wear. Price-wise, the difference between the two is minimal. My recommendation is to decide based on season and driving habits: opt for 30 in northern regions, and 40 for southern areas or mountainous terrains.

[KingAnn]

Back when I was helping out at the auto repair shop, I saw many cars with issues from using the wrong engine oil. The key difference between 5W30 and 5W40 lies in their high-temperature performance: 5W30 flows better at 100°C with a thinner oil film, making it ideal for daily commutes as it optimizes fuel efficiency and reduces wear. However, for frequent highway driving or heavy loads, 5W40's higher high-temperature viscosity provides stronger engine coverage, preventing oil burning due to thermal deformation. Both grades perform similarly in cold conditions, capable of starting at -30°C. Some German cars originally recommend 40, while Japanese models often use 30. During inspections, I noticed engines using 40 had slightly quieter operation. Ultimately, choose based on your vehicle’s manual and your driving intensity.

[DeRachel]

I'm just an average office worker who commutes daily in a small car. The 5W30 full synthetic oil is thinner, starts up quickly, and saves fuel, making it noticeably more fuel-efficient in city traffic jams. The 5W40 is slightly thicker, providing more stable acceleration on highways but consuming a bit more fuel. The difference lies in high-temperature viscosity: the 30 doesn't offer as strong protection as the 40 in hot weather, while the 40 can handle high loads and is more durable. I tried switching to 40 and felt the engine was quieter on long trips, but overall, the 30 suits my short-distance commuting better. The prices are similar, so I recommend checking the manual or consulting a repair shop for advice.