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The belief that an engine with high mileage needs “thicker” oil is still popular among some drivers. However, changing the viscosity grade contrary to the manufacturer’s recommendations can do more harm than good it can impair lubrication and component performance and accelerate engine wear.
“The engine already has high mileage, so the factory oil isn’t enough” - this approach seems logical, especially for cars with 200,000 or 300,000 kilometers on the odometer. The problem is that in practice, it often leads to the opposite of the intended result.
Adam Klimek, an expert at TotalEnergies, debunks this popular myth in a video:
Oil performs many more functions in the engine
In a modern engine, oil is responsible for more than just reducing friction. It also plays a role in the operation of many mechanisms and components, such as hydraulic tensioners, valve lifters, and variable valve timing systems. In turbocharged engines, it additionally cools and lubricates the turbocharger, which operates at very high temperatures.
For this reason, changing the viscosity “on your own” can negatively affect engine performance. A product with the wrong specifications may take longer to reach key lubrication points, perform poorly during cold starts, and lose its properties more quickly.
What do the symbols 5W-40 and 15W-40 mean?
Engine oil designations can be a source of confusion. In the case of 15W-40 oil, the first number with the letter “W” refers to the lubricant’s viscosity at low temperatures, while the second refers to its viscosity at high operating temperatures.
Although 5W-40 and 15W-40 oils have the same high-temperature viscosity, they behave differently during a cold start. 5W-40 oil flows through the system faster and reaches the areas requiring lubrication more efficiently. With 15W-40, this process takes longer, meaning the engine runs without optimal protection for a certain period of time.
This is particularly important because the greatest wear on the engine often occurs immediately after starting a cold engine
“Thicker” oil may only help superficially
There is some truth to the belief in the benefits of higher-viscosity oil. In heavily worn engines, especially those with worn cylinder liners, it can temporarily improve sealing, reduce oil consumption, or slightly quiet the engine’s operation. However, this does not mean that it is a beneficial solution for the engine. Such a product can simultaneously impair the lubrication of other mechanisms, increase the load on the lubrication system, and lead to overheating and faster oil degradation. The turbocharger is particularly sensitive to such changes, as it requires a stable and rapid flow of lubricant. As a result, using “thicker” oil may not extend the engine’s lifespan but could actually accelerate its failure.
Viscosity alone is not enough
When choosing oil, one should not rely solely on the viscosity grade. Two products, for example, both labeled 5W-30, can differ significantly in their properties and meet different automotive manufacturer standards. Therefore, it is worth checking the information on the packaging label before purchasing. That is where you can find data regarding certifications, quality standards, and manufacturer approvals that determine whether a given oil meets the requirements of a specific engine.
It is best to follow the manufacturer’s recommendations
High mileage should not be a reason to change the oil’s viscosity grade on your own. The safest solution remains using a product that meets the vehicle manufacturer’s requirements both in terms of viscosity and the required standards or approvals.
Information on the appropriate parameters can be found in the owner’s manual, the vehicle’s service manual, or the TotalEnergies oil finder: