Which oil is better — 5W-30 or 5W-40?

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      Engine oils 5W-30 and 5W-40 are suitable for most modern passenger cars due to their versatility and high levels of protection under heavy loads. These lubricants are designed for daily use in various conditions, which is relevant for residents of Russian regions with warm and harsh climates.

      Let’s look at how oils with different viscosity ratings differ and which one is better to pour into a car engine.

      5W-30 and 5W-40 oils: technical characteristics

      The low-temperature viscosity indices for oils in these classes are identical — 5W. This means both types provide effective engine starts during winter. Motor oils 5W-30 and 5W-40 have a pour point of −44 °C.

      The main difference between the two oils is high-temperature viscosity, indicated by the numeric index 30 or 40. The higher this number, the thicker the lubricant’s consistency will be inside the engine during and after warm-up.

      5W-30 and 5W-40 engine oils differ in composition, additive type, and additive properties. Depending on the manufacturer, kinematic viscosity can vary by 30–35%. When heated to 100 °C, the values range from 9.3–12.5 and 12.5–16.3 mm2/s for 5W-30 and 5W-40, respectively.

      Laboratory results

      To see the differences between 5W-30 and 5W-40 motor oils, let’s evaluate laboratory test results.

      5W-30 and 5W-40 oils have the same pour point and roughly identical kinematic viscosity values when heated to 100 °C. Higher dynamic viscosity characteristics are observed for 5W-40 when cooled to below-zero temperatures.

      Which oil is better for the engine

      There is no definitive answer to this question, as it all depends on the type and specifications of the particular engine. Manufacturers usually design engines to operate on a specific type of lubricant. For example, an oil recommended for a diesel engine may not be suitable for a gasoline engine, and vice versa. Some engines are recommended to use oils with increased high-temperature viscosity (5W-40). Other power units under similar operating conditions use less viscous lubricants that form a thin oil film (5W-30). The best solution in this case is the lubricant recommended by the vehicle manufacturer.

      Which oil is better for winter

      The pour point and flowability of 5W-30 and 5W-40 are practically the same, so during starting and warm-up the oils perform equally. The difference becomes noticeable when reaching operating temperature — above +100 °C, at which point the oils’ characteristics change:

      - 5W-30 becomes more fluid, forms a very thin oil film and quickly runs off metal surfaces, reducing internal resistance to moving parts and lowering fuel consumption;

      - 5W-40 thickens and forms a dense oil film, providing high protection of engine parts from wear.

      When choosing an oil it is important to consider the operating temperature conditions of the vehicle as well as the manufacturer’s recommendations.

      Which oil is better for a high-mileage car

      Engines with significant wear have increased clearances between the cylinder-piston groups compared with new units. For this reason it is important that the lubricant fully fills the clearances — this helps avoid dry friction and overheating of parts.

      For engines with mileage over 200–300,000 kilometers experts recommend using 5W-40 oil. Due to its higher viscosity and lower fluidity, the oil will create a thick oil film that compensates for increased clearances and provides an acceptable level of protection.

      Synthetic vs. semi-synthetic — what’s the difference

      According to the SAE standard governing oil characteristics, synthetic and semi-synthetic products of the same viscosity class are practically indistinguishable. Both synthetic and semi-synthetic 5W-30 oils demonstrate the same flow characteristics at low and high temperatures. For this reason the type of oil can be chosen at your discretion. The main thing is to prefer fluids that meet the technical requirements of the specific vehicle.

      Can 5W-30 and 5W-40 be mixed

      Mixing motor oils of different brands, formulations, and viscosities is permissible only in exceptional cases, for example to prevent so-called oil starvation of the engine. To bring the lubricant level up to the required amount, you can add oil of a different viscosity to the engine, but it must be of the same type (synthetic or semi-synthetic). It is highly desirable to use a lubricant from the same manufacturer. Be aware that when combining 5W-30 and 5W-40, the final viscosity and cold-weather performance of the lubricant will depend on the mixing ratios.

      You should not drive for too long on mixed oil — this can negatively affect the engine’s condition. Replace the oil completely at the first opportunity.

      Photo provided by NTK LLC.

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