We shall get to the point immediately. The “W” in engine oil viscosity stands for “winter.” It is part of the viscosity grading system established by the Society of Automotive Engineers (SAE) to classify motor oils according to their viscosity characteristics.

In a motor oil grade, such as 10W-40, the “10W” indicates the oil’s viscosity at low temperatures (winter conditions). Here’s what the “W” and the numbers mean:

1. Low-Temperature Viscosity (W)

   • The number before the “W” (e.g., 10W) represents the oil’s viscosity at 0°F (-17.8°C), reflecting how the oil performs in cold temperatures. Lower numbers indicate better flow at low temperatures, meaning the oil will be less thick and more capable of protecting the engine during cold starts.
   • But bear in mind that “cold” in temperate climates mean temperatures ranging from above -3 deg Celsius to below 18 deg Celsius.
   • Thus the “W” viscosity DOES NOT apply to tropical countries like Malaysia since our median temperature is 27 deg Celsius. Even the coldest places in Malaysia such as Cameron Highlands rarely see 15 deg Celsius.

1. High-Temperature Viscosity:

   • The number after the “W” (e.g., 40 in 10W-40) represents the oil’s viscosity at 212°F (100°C), which is roughly the operating temperature of an engine. Higher numbers indicate a thicker oil at high temperatures, providing better protection under heavy load and high temperatures.

Example: 10W-40 Oil

• 10W: Indicates the oil flows well at low temperatures, making it suitable for cold climates.
• 40: Indicates the oil maintains sufficient thickness to protect the engine at high operating temperatures.

This system helps ensure that the oil can provide adequate protection and performance under a wide range of operating conditions, from cold starts in winter to high-temperature running.

The reason why engine oils sold in Malaysia have both winter and summer grades is because these oils are also available in other countries, including those that have winter seasons.

The origins of “multigrade” engine oils

Engine oils used to be single-grade only. There are still single grade engine oils, but these are now rare. By single grade we mean, you would buy an oil with one viscosity, such as SAE 10, SAE 30 or SAE 40, and so forth.

So, you would use the lowest viscosity grade possible, such as SAE 5 or SAE 10, during winter months when everything is frozen solid. The “thin” oil keeps itself viscous so that you could start your engine. However, the oil will be too thin when the engine reaches its operating temperature.

On the other hand, you will need to swap out that winter oil to something “heavier” in the hotter months, such as SAE 40, SAE 50, etc. This is to keep the oil from getting too thin in the hot weather. However, this oil will turn into a block of gel in the winter months. Even starting on very cold mornings such as 5 deg Celsius is a chore as the oil is too thick.

As such, oil engineers managed to develop additives that makes an engine oil thin enough that it does not freeze in winter, and stays thick enough when the engine is hot. This gave birth to “multigrade” engine oils that we see today, such as the aforementioned SAE 10W-40 grade. Therefore, you could use only one oil throughout the year.