How are oils graded for use in engines?

• A. By brand and price
• B. By weight and type
• C. By series and viscosity
• D. By application and color

Answer: (C)

Oils are graded for use in engines primarily based on their viscosity and performance characteristics, which are critical for engine operation. Viscosity refers to the thickness of the oil and its ability to flow at different temperatures. This is vital because engines operate under various conditions, and the oil must be able to maintain its protective qualities in both cold and hot temperatures.

The grading system often follows standards established by organizations such as the Society of Automotive Engineers (SAE), which assigns numbers to oils indicating their viscosity grade, such as "10W-30." The “W” stands for winter, reflecting the oil's performance in cold weather, while the numbers indicate the oil's flow characteristics at different temperatures.

The choice that focuses on series and viscosity is correct because it encapsulates these essential properties that determine the suitability of oil for specific engine needs. Other options, while mentioning aspects of oil, do not reflect the technical standards used for grading oils effectively. For example, brand and price don't inherently relate to oil performance, while application and color are not standard criteria for oil classification.