Given all the options for motor oil options out there, choosing the right oil for your car might seem like a daunting task. While there is a mountain of information about the various oil choices, the first step is honestly quite simple: Look in your car’s manual.
The owner's manual for your car will list its recommended oil weight, whether that's a standard format like 10W-30 or something more unusual. That number refers to the viscosity (or thickness) of the oil that you should use. You should adjust which weight and type for the seasons and your expected use of the car, which we’ll explain below. For regular use in moderate temperatures, what’s listed in your owner’s manual is fine. Always choose an oil from a brand that displays the starburst symbol that indicates the oil has been tested by the American Petroleum Institute (API).
You’ll also notice a two-character service designation on the container. API's latest service standards are SP for gasoline engines and CK-4 for diesels. These letters are based on a group of laboratory and engine tests that determine the oil’s ability 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’s been tested under a current standard. As of this writing, that includes SP, SN, SM, SL and SJ for gasoline engines and CK-4, CJ-4, CI-4, CH-4 and FA-4 for diesels.
Those are the basics, but there's a whole lot more to the story than that.
Understand the Labels
These are the labels you'll find on every container of reputable motor oil. The API doughnut on the right tells you if the oil meets a current service rating. It also provides the SAE (Society of Automotive Engineers) viscosity number and tells you if the oil has passed the Resource Conserving test. The starburst symbol on the left indicates that the oil has passed the service tests listed in the other doughnut.
Viscosity refers to a fluid's resistance to flow. Most motor oils’ viscosity is rated based on how thick it is at zero degrees Fahrenheit (represented by the number preceding the W, which stands winter, as well as its thickness at 212 degrees (represented by the second number after the dash in the viscosity designation).
Motor oil becomes thinner and runnier as it heats up and thickens as it cools. Within reason, thicker oil generally maintains a better film of lubrication between moving parts and seals vital components of your engine better. With the right additives to help it resist thinning too much in the heat, an oil can be rated for one viscosity when cold and another viscosity when hot. The more resistant oil is to thinning, the higher the second number (10W-40 versus 10W-30, for example) will be, and that's good.
Meanwhile, in low temperatures, oil has to be resistant to excessive thickening so it can still flow properly to all the moving parts in your engine. Excessive thickness can make it more difficult to start the engine, which reduces fuel economy. If the oil is too thick, the engine requires more energy to turn the crankshaft, which is partly submerged in a bath of oil. A lower number is better before the W for cold-weather performance, so a 5W oil is typically what's recommended for winter use. However, synthetic oils can be formulated to flow even more easily when cold, so they are able to pass tests that meet the 0W rating.
Once the engine is running, the oil heats up, which is why a higher second number is especially important for extreme uses and hotter-running, more complicated engines.
Why So Many Oils?
Look in auto parts stores and you'll see oils labeled for all kinds of specific purposes: high-tech engines, new cars, higher-mileage vehicles, heavy-duty/off-road SUVs, and even cars from certain countries. You'll see a wide selection of viscosities.
If you read your owner's manual, you'll know what oil the vehicle’s manufacturer recommended to use when it was brand-new. The manual may include a reference to Energy Conserving or Resource Conserving oils, which means that the oil passed a fuel economy lab test against a reference oil. While that doesn’t always translate to better fuel economy, most leading brands have at least some viscosities that are labeled as such.
How to Choose Between Synthetic and Conventional Motor Oil
Premium Conventional Oil: This is the standard new-car oil. All leading brands have these oils, which are available in several viscosities and tested under the latest API service level. Automakers usually specify a 5W-20 or 5W-30 oil for colder temperatures, with a 10W-30 oil as optional for higher ambient temperatures. These three ratings cover most light-duty vehicles on the road. Even more important, though, is changing the oil and filter regularly. We recommend changing your oil every 4,000 miles or four months. The absolute minimum is twice 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 is done.
Full Synthetic Oil: Oils made for high-tech engines or heavy-duty uses, be it a Ford F-150 that tows frequently or a Chevrolet Corvette with the latest supercharged LS engine, are full of synthetic additives. These oils’ labels indicate whether they’ve passed stringent special tests for superior, longer-lasting performance in all the critical areas, from viscosity index to protection against deposits. They flow better at low temperatures and maintain peak viscosity at high temperatures. So, why shouldn't everyone use them? These oils are expensive and not every engine needs them. There may even be some features that your engine needs that synthetic oils don't have. Again, follow the guidance in your owner's manual.
Synthetic Blend Oil: These have a dose of synthetic oil mixed with organic oil, and are formulated to provide protection for somewhat heavier engine loads and high temperatures. This generally means they're less volatile, so they evaporate far less, which reduces oil loss and increases fuel economy. These oils are popular with drivers of pickups or SUVs who want extra protection for activities that put more stress on the engine, such as hauling heavy loads. They're also much less expensive than full synthetics—sometimes just pennies more than a premium conventional oil.
Higher-Mileage Oil: Today's vehicles simply last longer. If you prefer to pay off your car and run the mileage well into the six figures, you have another oil choice:, oils formulated for higher-mileage vehicles. Almost two-thirds of the vehicles on the road have more than 75,000 miles on the odometer. Consequently, oil companies identified this as an area of customer interest, and have new oils they're recommending for these vehicles.
When your vehicle is somewhat 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 they leak and may crack, especially at lower temperatures. You’ll need to check your oil levels more frequently and may need to top off your oil between oil changes.
Higher-mileage oils are formulated with conditioners that flow into the pores of the engine seals to restore their shape and increase their flexibility. Most rubber seals are designed to swell just enough to stop leaks, and oil refiners pick their "reswelling" ingredients carefully. Valvoline showed us the performance data of one of their seal conditioners that caused most seal materials to swell while reducing the swelling of one seal material that tended to expand too much from the ingredients found in some other engine oils.
You also may have noticed some loss of performance and engine smoothness due to engine wear on your higher-mileage vehicle. These higher-mileage oils also have somewhat higher viscosities. Even if the numbers on the container don't indicate it, there's a fairly wide range for each viscosity rating and the higher-mileage oils sit at the top of each range. They may also have additives to improve their viscosity index. The result? They seal your pistons better against their cylinder walls and won't leak as much through larger engine bearing clearances that have worn down with time. They also may have a higher dose of antiwear additives to try to slow that wear process.
If you have an older vehicle, these features may mean more to you than what you might get from a full synthetic at a fraction the price.
An oil’s resistance to thinning in hotter temperatures is called the viscosity index. Although a higher second number is good, the oil also has to be robust, lasting for thousands of miles until the next oil change. Oil tends to lose viscosity from shear, which is the sliding motion in the tight clearances between metal surfaces,such as those found in bearings. So, resistance to viscosity loss—called shear stability—is necessary to enable the oil to maintain the lubricating film between those parts.
Unlike antifreeze, 95 percent of which is made up of one base chemical (typically ethylene glycol), petroleum-based engine oil contains a mixture of several different types of base oils—some of which are more expensive than others. Oil companies typically pick from a selection of five groups, each of which is produced in a different way and in different viscosities. The more expensive groups are more highly processed, in some cases with methods that produce a lubricant that can be classified as a synthetic. The so-called full synthetics contain chemicals that may be derived from petroleum but are so altered that they're not considered natural oil anymore. For example, one custom blend contained 10 percent polyalphaolefins (PAO), which is the most common type of chemical used as the primary ingredient in a full synthetic oil.
The base oil package in any oil makes up anywhere from 70 to 95 percent of the mix; and the rest is composed of additives. An oil with just 70 percent base oils isn’t necessarily better than one with 95 percent base oils. Some base oils have natural characteristics or ones that derive from their processing, which reduce or eliminate the need for additives. Although some additives make improvements lubrication, they don't necessarily have great lubricity on their own.
The ingredients in an additive package differ in cost, but price is just one factor. Some additives work better in certain combinations of base oils. Likewise, some less expensive base oils are a good choice for a blend because of the way they perform with popular additives. Bottom line: Every motor oil has a recipe. Refiners come up with a list of objectives based on the needs of their customers (including the carmakers themselves) and formulate oils to meet those goals as best they can.
Keeping an oil from thinning as it gets hot while it takes a beating from engine operation is one thing, but it's also important to keep oil from becoming too thick. Using less volatile premium base oils to prevent evaporation is one approach. Evaporation of the base oil package not only increases oil consumption—it results in thicker oil, which decreases fuel economy.
Oil companies’ use of additives is another approach to improving and maintaining oil performance. High engine temperatures combine with moisture, combustion byproducts (such as unburned gasoline), rust, corrosion, engine-wear particles and oxygen to produce sludge and varnish, which can gum up and damage the engine. Additives help maintain good lubrication by minimizing sludge and varnish. Here are the main categories of additive ingredients and why they're important:
• Viscosity-index improvers: These reduce the oil's tendency to thin with increasing temperature.
• Detergents: Unlike the kind you use to wash clothes, detergents in oil don't scrub engine surfaces. They do remove some deposits—primarily solids. However, their main purpose is to keep surfaces clean by inhibiting the formation of high-temperature deposits, rust, and corrosion.
• Dispersants: These disperse solid particles by keeping them in a solution so they don't come together to form sludge, varnish or acids. Some additives work both as detergents and dispersants.
• Antiwear agents: Sometimes the lubricating film created by oil breaks down, so antiwear agents have to protect the metal surfaces. A zinc and phosphorus compound called ZDDP is a long-used favorite, along with other phosphorus (and sulphur) compounds. If you must know, ZDDP stands for zinc dialkyl dithiophosphate.
• Friction modifiers: These aren't the same as antiwear agents. They reduce engine friction and thus, can improve fuel economy. Graphite, molybdenum and other compounds are used for this.
• Pour-point depressants: Just because a 0 degrees Fahrenheit viscosity rating is low doesn't mean that oil will flow readily at low temperatures. Oil contains wax particles that can congeal and reduce flow, so these additives are used to keep it flowing in the cold.
Antioxidants: With tighter emissions regulations resulting in higher engine temperatures, antioxidants are needed to prevent oxidation that thickens the oil. Some of the additives that perform other functions also serve this purpose, such as the antiwear agents.
• Foam inhibitors: The crankshaft whipping through the oil in the oil pan causes oil to foam. Oil foam is not as effective a lubricant as a liquid stream, so oils have foam inhibitors that cause the foam bubbles to collapse.
• Rust or corrosion inhibitors: These protect metal parts from acids and moisture.
More Is Not Better
You can't necessarily improve an oil by putting in more additives. In fact, you can make things worse. For example, sulphur compounds have antiwear and antioxidation characteristics, but they can reduce fuel economy and reduce the effectiveness of your catalytic converter. Too much of a specific dispersant can affect catalyst performance and reduce fuel economy. Antiwear and friction-reducing additives also may have ingredients that could affect catalyst performance, such as sulphur, which companies are being pressured to use less. Adding tToo much of some detergents can also affect antiwear characteristics.
Don't Forget the Filter
Oil filters are a completely different,albeit related, subject when it comes to changing your oil. Again, it's always best to consult your owner’s manual for the type of filter required. Some aftermarket filters are larger, so make sure you have some extra oil if you use one.
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