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This article originally appeared the the Castrol Blog.

DIFFERENT OIL FOR DIFFERENT WEATHER
Donning a hat and gloves in the chill of winter or a pair of shorts in the summer months are not the only changes to consider when the temperature dips or rises. Outside temperature is an important factor when choosing the right motor oil. Winter cold or summer heat affects the rate at which motor oil flows. The measure of this flow is the viscosity of the motor oil.

THICK AND THIN
Motor oil viscosity is expressed as a number. Thicker motor oil has a higher viscosity number. Thinner motor oil has a lower number. Multiple viscosity oils carry with them a range of numbers—10W–40, for example. In this case the 10 is a representation of the motor oil’s viscosity when cold and is determined from tests conducted at temperatures below zero degrees Fahrenheit. The W stands for Winter. The 40 represents the viscosity of the oil when tested at an engine operating temperature of 212 degrees Fahrenheit.

COLD WEATHER STARTUP
The cold weather measure of motor oil viscosity is crucial to the ongoing survival of an automobile engine during the first few seconds of extreme cold weather operation. The old saying “Slower than molasses in January” also applies to motor oil. Engine damage can occur if motor oil is too thick due to cold and is unable to flow in between engine parts during a cold weather startup. Which viscosity of oil to use depends on the weather conditions where the car is operated along with manufacturer recommendations.

STRUNG TOGETHER
Single grade motor oil has only one number to represent its viscosity. Single viscosity motor oils are rarely used in today’s engines. Polymers are added to a single viscosity oil to make one oil behave like many. In this way a 0W–30 motor oil will flow like a 0W viscosity oil when cold and a 30 viscosity oil when hot. Automobile manufacturers have shifted towards lower viscosity oils for fuel efficiency. Thicker or higher viscosity oil requires more energy to move than its thinner cousin.

CHART VISCOSITY
The solution for deciding which viscosity of oil to use for what time of year is in the vehicle owner’s manual. Switching from a 10W–30 to a 5W–30 during the winter months might be required if the vehicle is operated in an area that typically sees the thermometer dip below the zero mark. This information is in the owner’s manual. Oil viscosity requirements are similar to tire pressure. The vehicle manufacturer sets the recommended tire pressure regardless of which brand of tire is used. Always use only the viscosity recommended by the manufacturer.

SEVERE DUTY
With the correct viscosity determined the next choice in motor oil is type. The best strategy is to follow what the vehicle owner’s manual recommends. Higher mileage blends can benefit engines over 75K miles. Synthetic blends and full synthetics can bring additional protection and often flow more easily during severe cold then conventional oils. Remember that stop–and–start driving during winter or summer months qualifies as severe duty in most manufacturer maintenance schedules. Changing the oil and filter according to this schedule can save expensive engine repairs.