How to Understand Motor Oil Weight

Motor oil is rated by how well it flows under both high and low temperatures.

While some people change their vehicle's motor oil themselves to save money, they may not fully understand the oil's SAE (Society of Automotive Engineers) ratings. Oil weight is a measurement of the oil's viscosity, or thickness, related to its performance at two temperature extremes of 0 degrees and 210 degrees Fahrenheit. Oil is heated, or cooled, to the specified temperature and its viscosity is determined by the oil's rate of flow. Follow the manufacturer's recommendations when selecting a motor oil for your vehicle.

Instructions

1. Locate the numbers on the bottle's label. A higher number indicates a thicker oil. An SAE 40 weight oil has a greater viscosity and a slower flow rate than an SAE 10. Higher numbers offer better protection at higher operating temperatures.

2. Determine if the SAE number is followed by the letter "W," which stands for "winter." An SAE 10W oil is rated for cold-weather start-ups.

3. Look for hyphens between the numbers, such as 10W-30. These oils have a multi-viscosity rating and are referred to as all-season motor oils because they are designed to function at varying temperatures.

Tips & Warnings

• Many vehicles will have the manufacturer's recommended oil weight imprinted on the engine's oil-fill cap.

References

• Auto Education: Oil Lubrication
• AA1 Car: What Every Motorist Should Know About Motor Oil

Resources

• AMS Oil: Motor Oil Viscosity Grades