What is the thickest motor oil?

The thickest motor oil is typically called 20W-50. The first number, 20, refers to the oil’s cold-start performance. The “W” stands for winter, so this is an oil designed to work in colder temperatures. The second number, 50, refers to the oil’s viscosity at operating temperature. A higher second number, like 50, means the oil is thicker and provides better protection at higher temperatures and under heavy loads. Thicker oils may be recommended for older engines with more wear or racing engines, but it is important to use the oil viscosity recommended by the manufacturer for proper engine performance and longevity.