How to Select the Correct Viscosity Oil for Your Car

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Choosing the correct viscosity oil for your car’s engine requires some basic understanding of oil viscosity and its ratings. Viscosity is a measurement of oil’s weight, and actually describes how thick the oil is. Oil tends to thicken, or increase in viscosity, at low temperatures and thin out, or decrease in viscosity, at higher temperatures. This seems natural to most people. We expect oil to thin out as it heats up and to thicken as it cools down.

Motor oil however, thickens as it heats up and thins as it cools down. This is counter-intuitive, and tends to be the most confusing aspect of understanding motor oil viscosities. Refiners place special additives in the oil to make it thicken when heated. The thickened oil clings to your engine’s internal parts, increasing the lubrication provided by the oil.

The SAE (Society of Automotive Engineers) uses a numerical designation to rate oil’s viscosity. The lower the SAE number the lower the oil’s viscosity. As the SAE rating increases, so does the viscosity of the oil. In multi-grade oils two different numerical grades are given. For example, the rating “10W-30” means the oil’s viscosity is “10” in cold temperatures, and “30” in warm temperatures. The “W” in a 10W-30 rating means winter. Therefore, a 10W-30 rating actually translates to a viscosity rating of 10 in the winter, and 30 during warmer weather.

Choosing the right viscosity oil for your car is a simple matter of considering three important facts. First, what oil viscosity does the vehicle’s manufacturer recommend? Second, do you drive long distances, or do mostly stop-and-go driving? Third, what weather conditions do you normally drive in?

Manufacturer’s Recommendation

Check your car’s owner manual or contact a local dealership and ask what viscosity oil is recommended for your vehicle. If your vehicle is under warranty, use the oil they recommend. Failure to do so can result in the denial of claims under the warranty. If the warranty is no longer in effect, you can exercise your personal preferences. However, if you are unsure what to use always defer to the manufacturer’s recommendation.

Driving Style

Consider your driving style. If you drive mostly highway miles, you can use oil with a lower viscosity rating. For example, you could replace your 10W-30 oil with 5W-30. If you do more stop-and-go driving, use higher viscosity oil. An example would be to replace your 10W-30 with 20W-50 oil. The 20W-50 oil provides better protection for engines switched on and off several times a day.

Weather Conditions

Think about the weather conditions you will be driving in. For very cold weather, a viscosity rating of 5W-30 provides maximum protection. When driving in temperatures running from 0 to 60 degrees Fahrenheit, a rating of 10W-30 offers excellent all around protection. For driving in extremely hot conditions, 20W-50 oil offers the best protection.


For most drivers it is best to follow the recommendations of the vehicle’s manufacturer. Far more important than oil viscosity is regular oil and filter changes. Keeping metal particulates out of your engine’s oil will do more to prolong engine life than the viscosity of oil you choose.


Ultimately, selecting the right viscosity oil is a decision based on your personal situation. For new or nearly new cars, use the manufacturer’s recommendation. Remember to consider your driving style. Finally, adjust for the type of weather you most often drive in.

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