Gear oil viscosity is typically read by understanding the Society of Automotive Engineers (SAE) grading system, particularly for multigrade oils, which use a specific numerical code.
Understanding SAE Gearbox Oil Grades
Gear oil, much like engine oil, is assigned an SAE grade number that signifies its viscosity characteristics at different temperatures. A common and important system for multigrade SAE gearbox oil grades involves two numbers separated by the letter 'W', such as 75W90 or 75W140.
Deciphering the Multigrade Code
To read these grades, break down the components:
- The number before the 'W' (e.g., 75 in 75W90) relates to the oil's performance and flow at 0°C (representing cold temperatures). A lower number here indicates better fluidity in cold conditions.
- The 'W' stands for "Winter," signifying that the number preceding it indicates cold-temperature performance.
- The number after the 'W' (e.g., 90 in 75W90, or 140 in 75W140) specifies the oil's viscosity at a standard operating temperature of 100°C.
The Key Rule for Viscosity
A fundamental principle for interpreting these numbers is directly related to viscosity itself:
- As with engine oils, the higher the number, the greater its viscosity.
This means a grade of 140 is significantly more viscous than a grade of 90 at 100°C. Similarly, while both 75W90 and 80W90 have a grade of 90 at 100°C, the 75W90 offers better cold-weather flow (lower number before 'W', less viscous in the cold).
Practical Example
Let's look at two common multigrade gear oils:
- 75W90: This oil performs like a 75-grade oil in cold temperatures (0°C) and a 90-grade oil at operating temperatures (100°C).
- 75W140: This oil performs similarly to 75W90 in the cold (both are 75W) but is much thicker (more viscous) at operating temperature, performing like a 140-grade oil.
Understanding these numbers helps in selecting the appropriate gear oil recommended by the vehicle manufacturer for optimal performance and protection under specific operating conditions and temperatures.
