How Do I Know What Type of Oil My Car Needs?

Given all the options for engine oil options out there, choosing the right oil for your car might seem like a daunting task. While there’s a lot of information about the different oil choices, the first step is quite simple: look at your car manual.

The owner’s manual for your car lists the recommended oil weight, whether it’s a standard format like 10W-30 or something more unusual. This number refers to the viscosity (or thickness) of the oil that you should use. You should adjust the weight and type for the seasons and your expected use of the car, which we’ll explain below. What is listed in your owner’s manual is fine for regular use at moderate temperatures. Always choose an oil from a brand that displays a starburst symbol that indicates that the oil has been tested by the American Petroleum Institute ( API).

You will also notice a two-character service name on the container. The latest API service standards are SP for gasoline engines and CK-4 for diesel engines. These letters are based on a group of laboratory and engine tests that determine the ability of the oil to protect the engine from wear and high-temperature deposits and sludge. API has a full list of these standards here in case you’re curious, but make sure you’re buying an oil that has been tested to the current standard. As of this writing, it includes SP, SN, SM, SL and SJ for gasoline engines and CK-4, CJ-4, CI-4, CH-4 and FA-4 for diesel engines.


Viscosity refers to the resistance of the fluid to the flow. Most of the engine oils are rated on the basis of their thickness at zero degrees Fahrenheit (represented by the number preceding the W, which stands for winter, as well as their thickness at 212 degrees Fahrenheit (represented by the second number after the dash in the name of viscosity).

Motor oil is getting thinner and runner as it heats up and thickens as it cools. Within reason, the thicker oil generally maintains a better lubricating film between moving parts and sealing vital components of your engine. With the right additives to help it withstand thinning too much in the heat, the oil can be rated for one viscosity when it is cold and another viscosity when it is hot. The more resistant the oil is to thin, the higher the second number (10W-40 versus 10W-30, for example) will be, and that’s good.

In the meantime, at low temperatures, the oil must be resistant to excessive thickening so that it can still flow properly to all moving parts of your engine. Excessive thickness may make it more difficult to start the engine, which reduces fuel economy. If the oil is too thick, the engine needs more energy to turn the crankshaft, which is partially submerged in an oil bath. The lower number is better than the W for cold-weather performance, so the 5W oil is usually recommended for winter use. However, synthetic oils can be formulated to flow even more easily when cold, so that they can pass tests that meet the 0W rating.

Once the engine is running, the oil heats up, which is why higher seconds are particularly important for extreme use and hotter-running, more complicated engines.

How to Select Between Synthetic and Conventional Motor Oil

Premium Conventional Oil

It’s the standard new-car oil. All leading brands have these oils, which are available in a number of viscosities and tested at the latest API service level. Automakers usually specify a 5W-20 or 5W-30 oil for cooler temperatures, with an optional 10W-30 oil for higher ambient temperatures. These three ratings cover most of the light-duty vehicles on the road. However, it is even more important to change the oil and filter on a regular basis. We recommend that you change your oil every 4,000 miles or four months. The absolute minimum is two times a year. If your car has an electronic oil change indicator on its instrument cluster, follow its guidance instead and be sure to reset it once your oil change has been made.

Full Synthetic Oil

Oils made for high-tech or heavy-duty engines, be they Ford F-150s that frequently tow or Chevrolet Corvettes with the latest supercharged LS engines, are packed with synthetic additives. These oil labels indicate whether they have passed strict special tests for superior, longer-lasting performance in all critical areas, from the viscosity index to the deposit protection. They flow better at low temperatures and maintain maximum viscosity at high temperatures. So, why shouldn’t they all be used? These oils are expensive, not every engine needs them. There may even be some features your engine needs that synthetic oils don’t have. Again, follow the instructions in the owner’s manual.

Synthetic Blend Oil

These contain a dose of synthetic oil mixed with organic oil and are formulated to provide protection for slightly heavier engine loads and high temperatures. This generally means that they are less volatile, so that they evaporate much less, which reduces oil losses and increases fuel economy. These oils are popular with pick-up or SUV drivers who want extra protection for activities that put more stress on the engine, such as heavy load transport. They’re also much cheaper than full synthetics — sometimes just pennies more than conventional premium oil.

Higher Mileage Oil

Today’s vehicles just last longer. If you prefer to pay off your car and run the mileage well into the six figures, you have another choice of oil: oils formulated for higher-mileage vehicles. Almost two thirds of the vehicles on the road have more than 75.000 miles on the odometer. As a result, oil companies have identified this as an area of customer interest and have new oils that they are recommending for these vehicles.

If your vehicle is a little older and has considerably more mileage, you may notice a few oil stains on the garage floor. Engine seals, such as those around the crankshaft, may have hardened and lost their flexibility, so that they leak and crack, especially at lower temperatures. You will need to check your oil levels more often, and you may need to top up your oil between changes.

Higher-mileage oils are formulated with conditioners that flow into the pores of the engine seals in order to restore their shape and increase their flexibility. Most rubber seals are designed to swell just enough to stop leakage, and oil refiners carefully pick up their “reswelling” ingredients. Valvoline showed us the performance data of one of its seal conditioners, which caused most seal materials to swell while reducing the swelling of one seal material, which tended to expand too much from the ingredients found in some other engine oils.

By Jon ‘ShakataGaNai’ Davis, CC BY 3.0,

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