While researching what the viscosity numbers on various oils mean, to see how various oils are compatible with various engine requirements, I thought it would be prudent to trust the oil manufacturers' website data more than joe-next-door's-DIY-servicing website.
Alas even they can not get it right all the time, as can be seen here in an extract from Total's website. Let's see who can spot the errors in this article:
What does 5W40 actually mean?
These two numbers in the name signify the performance of the oil under two different conditions, during start-up and when the car is in normal operating condition. The “W” stands for “winter” and the number preceding it indicates the oil’s viscosity (or thickness) at low temperatures (hence the W) when you start the vehicle. The number after the W indicates viscosity at higher temperatures (150oC on average), when the engine is actually running. The higher the second number is, the thicker the oil is at that defined temperature condition.
5W40 motor oil has a winter viscosity grade of 5, meaning it’s less viscous at very low temperatures, compared to for example a 10W40. At low temperatures (during start-up), a lower viscosity oil can move between the moving parts quickly as it flows more easily. This forms a protective oil film faster when the surfaces start to move compared to a monograde. This is an advantage of a multi-grade oil that a monograde (those that only have one number) does not provide.
It also has a hot viscosity grade of 40, which means it is just as viscous as 10W40 at high temperatures, but less viscous than oils such as 15W50. As the temperature increases, the existing protective oil film will become thinner, therefore a higher viscosity oil will provide a thicker protective film.
Why does the viscosity of motor 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 since they don’t separate moving surfaces properly. Viscosity decreases (becomes thinner) as temperatures increase, and the oil’s viscosity increase (thickens) as the temperature drops.
Extracted from here: https://www.total.co.za/blog/when-should-i-use-5w40-motor-oil
Alas even they can not get it right all the time, as can be seen here in an extract from Total's website. Let's see who can spot the errors in this article:
What does 5W40 actually mean?
These two numbers in the name signify the performance of the oil under two different conditions, during start-up and when the car is in normal operating condition. The “W” stands for “winter” and the number preceding it indicates the oil’s viscosity (or thickness) at low temperatures (hence the W) when you start the vehicle. The number after the W indicates viscosity at higher temperatures (150oC on average), when the engine is actually running. The higher the second number is, the thicker the oil is at that defined temperature condition.
5W40 motor oil has a winter viscosity grade of 5, meaning it’s less viscous at very low temperatures, compared to for example a 10W40. At low temperatures (during start-up), a lower viscosity oil can move between the moving parts quickly as it flows more easily. This forms a protective oil film faster when the surfaces start to move compared to a monograde. This is an advantage of a multi-grade oil that a monograde (those that only have one number) does not provide.
It also has a hot viscosity grade of 40, which means it is just as viscous as 10W40 at high temperatures, but less viscous than oils such as 15W50. As the temperature increases, the existing protective oil film will become thinner, therefore a higher viscosity oil will provide a thicker protective film.
Why does the viscosity of motor 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 since they don’t separate moving surfaces properly. Viscosity decreases (becomes thinner) as temperatures increase, and the oil’s viscosity increase (thickens) as the temperature drops.
Extracted from here: https://www.total.co.za/blog/when-should-i-use-5w40-motor-oil
