What do the SAE ratings on motor oil mean?

SAE ratings on motor oil have to do with the viscosity of the oil. The viscosity is the oil's resistance to flow, or an oil's speed of flow as measured through a device known as a viscometer. The higher the SAE, the thicker the oil and the slower it will flow.

Oils that have a number like 5W30 or 10W30 have been tested at a colder temperature. These oils will flow at the fist number's rate at colder temperatures and the second number's rate at higher temperatures.

SAE means Society of Automotive Engineers