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The differences between 5w30 and 5w40 oils
For 5w30 oil:
Kinematic viscosity is between 9.3-12.5mm²/s
Dynamic viscosity is 2.9mPas.
For 5w40 oil:
Kinematic viscosity is between 12.5-16.3mm²/s
Dynamic viscosity is 3.5mPas.
In practice, this means that each oil is just as viscous at lower temperatures and will remain viscous at -30°C, however 5w40 oil outperforms 5w30 oils at higher temperatures, being effective up to ambient temperatures of 50°C, as opposed to 30°C.
5w40 and 5w30 motor oils have winter viscosity grades of five, meaning they are effective down to temperatures of -30°C, and are more viscous at very low temperatures, compared to, say, 10w40 oils.
Resultantly, 5w40 oils are likely to be used in higher-performance engines that have higher operating temperatures and loads than the norm – those driven in urban environments, that commonly sit in traffic or driven at particularly high speeds, for example.
If your vehicle is not designed to be driven in a high-performance manner, or you drive at a more considered, environmentally friendly pace, 5w30 oils are a good choice. These are commonly better for certain engine components such as oil pumps, can clean the engine better, and have longer lifespans due to having a lower temperature range.
What about 5w40 or 10w40 engine oil?
The choice between 5w40 motor oil and an alternative like 10w40 depends on the winter temperature in your region. 5w40 is better for starting your engine at low temperatures as it flows more easily in this environment. When the engine is running and hot, the differences are negligible; 5w40 and 10w40 both have high viscosities and are equally effective in protecting against engine wear.