Which of the following grades of oil is considered the thickest?

The correct answer identifies SAE 10W-40 as the thickest oil grade among the options presented. The numbering system in the SAE grading for motor oil indicates the oil's viscosity—the ability to flow—at different temperatures. The first number represents the oil's viscosity at low temperatures, while the second number reflects its viscosity at high temperatures.

In this case, the "40" designation indicates that at high temperatures, SAE 10W-40 has a higher viscosity than the other listed grades, meaning it is thicker and can provide better film strength and protection under heavy use and high-load situations. This characteristic makes it particularly suitable for high-performance engines or environments that require additional lubrication stability.

While the other oils listed, such as SAE 5W-30, SAE 10W-30, and SAE 20W-50, serve their purposes well, they do not provide the same high-temperature viscosity as SAE 10W-40, particularly at operating temperatures. SAE 20W-50 is a thicker oil than SAE 10W-40 at high temperatures but has a "20W" designation that indicates a lower viscosity at cold temperatures compared to SAE 10W-40. Therefore, when considering overall thickness and performance at high temperatures