What Is the Difference in 10w-40 Car Oil and 5w-30 Oil?

The first number in the motor oil's name describes the oil's cold viscosity. Cold viscosity refers to the thickness of the oil upon a cold start of the engine, before the pistons and engine block have warmed up from ignition and combustion. Motor oil's cold viscosity is important because it dictates the lubrication during the hardest part of the combustion cycle, the startup and initial ignition in gasoline engines or combustion in diesel-fueled engines. A 10w-40 motor oil is a thicker oil at startup than a 5w-30 motor oil. Therefore, 10w-40 oil clings to the engine's moving parts more than the lower viscosity 5w-30 oil.

The second number in the motor oil's name describes the viscosity of the oil when the engine is warm and the oil reaches operating temperature. This warm viscosity is always going to be heavier because the engine requires a higher viscosity oil to provide ample lubrication through the broad range of the engine cycle and temperatures. The 10w-40 motor oil clings to the engine's moving parts more than a 5w-30 motor oil does at higher temperatures; this means the 10w-40 is the thicker of the motor oils when the engine is running.

Automobile manufacturers often urge their engineers to increase fuel economy in order to meet government regulations and provide an edge against competitors in the marketplace. Thinner motor oil, such as 5w-30, provides less friction in the engine, resulting in increased fuel economy. Automobile manufacturers may recommend thinner motor oil for this reason, although the increased fuel economy comes at the cost of increased engine wear.

The repeated motion of the engine's cylinders moving in and out of the engine block can create wear over the engine's lifespan. As the engine wears, the fit between the cylinders and block becomes looser, which can significantly increase wear and decrease usable life as the engine develops more gaps between the moving parts. A thicker motor oil, such as 10w-40, provides a more solid lubricant to compensate for the excess wear of high-mileage engines. Many experienced mechanics recommend shifting to a higher viscosity motor oil later in the engine's lifespan in order to compensate for these imperfections.