Short Answer
Complete Explanation
The second number in a motor‑oil grade, such as the “30” in 10W‑30, represents the oil’s viscosity at 100 °C (212 °F), which approximates the temperature of a running engine. This figure is part of the SAE (Society of Automotive Engineers) viscosity classification system and indicates how thick the oil remains when the engine is hot.
- Viscosity at 100 °C:
The number denotes the oil’s kinematic viscosity measured at 100 °C, expressed in centistokes (cSt). - Upper Temperature Limit:
It shows the oil’s ability to maintain sufficient film thickness at high operating temperatures. - Compatibility with Engine Design:
Manufacturers specify a minimum second number to ensure proper lubrication for bearing clearances and oil pump performance.
Common Misconceptions
The second number indicates the oil’s thickness when the engine is cold.
Cold‑temperature viscosity is represented by the first number (with “W”). The second number applies to hot‑engine conditions.
A higher second number always provides better protection.
While a higher number means thicker oil at operating temperature, it can reduce fuel efficiency and may not be suitable for all engines.
FAQ
Why does the first number have a “W”?
The “W” stands for winter and indicates the oil’s viscosity at low (cold) temperatures, measured at 0 °C or -30 °C depending on the grade.
Can I use oil with a higher second number than recommended?
Using a higher second number results in thicker oil at operating temperature, which may protect better under extreme heat but can increase friction, reduce fuel efficiency, and affect oil flow in engines designed for lower viscosity.
How does the second number affect fuel economy?
Thicker oil (higher second number) creates more internal resistance, which can slightly lower fuel economy. Modern engines are often optimized for specific viscosity ranges to balance protection and efficiency.
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