How to Choose the Best Motor Oil for Your Climate

The property of motor oil that maintains a lubricating film between moving parts is called its viscosity. The viscosity of a liquid can be thought of as its "thickness" or resistance to flow. The viscosity needs be high enough to maintain a film of lubrication, but low enough that the oil can flow around the engine parts well enough to keep them coated under all conditions.

The viscosity index tells you how much the oil's viscosity changes as temperature changes. A higher viscosity index means that the viscosity changes less with temperature than an oil with a lower viscosity index.

If you live in New England, or any other location where the temperatures are cold, your motor oil must be able to flow at cold temperatures to lubricate your engine components at start up. Another important property of motor oil is its pour point, which is gives you an idea of the lowest temperature at which the oil could still be poured. The lower the pour point temperature of the oil, the more desirable the oil is when it comes to starting your engine at cold temperatures.

Your automobile manufacturer should recommend the proper motor oil for your car, and location. Some European manufacturers do not specify the oil viscosity at all. The following table gives you an idea of the acceptable outdoor temperature range, in Fahrenheit, for various grades of motor oil.

20W-20 is good from 20 to 100° F
20W-40 is good from 20 to 100° F
20W-50 is good from 20 to 100° F
10W-30 is good from 0 to 100° F
10W-40 is good from 0 to 100° F
10W is good from 0 to 60° F
5W-30 is good from -20 to 60° F
5W-20 is good from -20 to 0° F

The SAE numbering system for multi-grade motor oils includes two grade numbers; for example, 10W-30 is common multi-grade oil. The first number associated with the W ('W' is use to designate Winter grade, not Weight) is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as well as single-grade SAE 10 motor oil. "5W" can be pumped at a lower temperature than "10W" and "0W" can be pumped at a lower temperature than "5W". The second number, 30, means that the viscosity of this multi-grade oil, at an operating temperature of 212°F (100°C), is comparable to the viscosity of a single-grade 30 motor oil at the same temperature.

The values listed above are for average conditions, and may vary depending on the equipment in service.  Manufacturers specifications should always be taken over general references like this.  Of course, you should never run your engine when it is low on motor oil, or when the oil light is on.

Always make sure that you are using the right motor oil for your vehicle, and location.  If your car motor vehicle manufacturer doesn't tell you which oil to use, you should be safe using this guide.