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5W-40: choosing the right engine oil for your car

There are two important factors to consider when comparing and choosing engine oils—how cold does it get in winter and what the manufacturer recommends for your engine. Ideal for starting engines at low temperatures and effective at high engine temperatures, 5W-40 engine oils can be used in most conditions.

What does 5W-40 actually mean?

The “W” stands for “winter,” the number preceding it indicates the oil’s viscosity (or thickness) at low temperatures (hence the W), and the number after it indicates viscosity at higher temperatures (150oC on average), when the engine is actually running.

5W-40 engine oil has a winter viscosity grade of five, meaning it’s less viscous at very low temperatures, compared to, say, 10W-40. It has a hot viscosity grade of 40, which means it is just as viscous as 10W-40 at high temperatures but less viscous than oils such as 15W-50.

Why does the viscosity of engine oil matter?

An oil’s viscosity is its resistance to flow. Oil that’s too thick won’t flow properly through your engine. Oil that’s too thin won’t protect moving parts from wear and tear. Viscosity increases as the temperature drops and decreases as the temperature rises. 

When selecting a particular grade of engine oil, you face a tradeoff between performance and protection. An oil that’s more viscous while your engine is running will do a better job of protecting engine components. But an oil that’s too thick will also be somewhat of a drag on engine performance. That’s why it’s important to consult your engine manufacturer about exactly which grade of oil is appropriate.

So do I need 5W-40 or 10W-40?

The choice between 5W-40 engine oil and an alternative like 10W-40 depends on the winter temperature in your region. 5W-40 is better for starting your engine at low temperatures as it flows more easily at these temperatures. When the engine is running and hot, 5W-40 and 10w40 have high viscosities and are equally effective in protecting against engine wear.