Given the extensive array of motor oil options available today, choosing the correct oil for your vehicle can indeed appear to be a formidable undertaking. While there’s a wealth of information concerning various oil selections, the initial and simplest step is to consult your car’s owner’s manual.
Your car’s owner’s manual will specify its recommended oil weight, whether that’s a conventional format like 10W-30 or something less common. This number indicates the oil’s viscosity (or thickness) that you should use. You might need to adjust the weight and type based on the seasons and your anticipated vehicle usage, which we will elaborate on below. For routine driving in temperate conditions, the oil specified in your owner’s manual is perfectly suitable. Always opt for an oil from a brand that features the starburst symbol, signifying that the oil has been tested by the American Petroleum Institute (API).
You’ll also observe a two-character service designation on the container. API’s most recent service standards are SP for gasoline engines and CK-4 for diesels. These alphanumeric codes are derived from a series of laboratory and engine tests that assess the oil’s capacity to safeguard the engine from wear, high-temperature deposits, and sludge. API provides a comprehensive list of these standards here if you’re interested, but ensure you are purchasing an oil that complies with a current standard. As of the time of this writing, this includes SP, SN, SM, SL, and SJ for gasoline engines, and CK-4, CJ-4, CI-4, CH-4, and FA-4 for diesels.
Beyond the Basics: Understanding Labels
SAE
Motor oil labels typically feature specific markings. The API doughnut on the right side of the label indicates if the oil satisfies a current service rating. It also displays the SAE (Society of Automotive Engineers) viscosity number and confirms if the oil has passed the Resource Conserving test. The starburst symbol on the left signifies that the oil has successfully undergone the service tests outlined in the API doughnut.
Viscosity
Viscosity refers to a fluid’s inherent resistance to flow. The viscosity of most motor oils is rated based on their thickness at zero degrees Fahrenheit (represented by the number preceding the ‘W’, which stands for winter) and their thickness at 212 degrees Fahrenheit (represented by the second number following the dash in the viscosity designation).
Motor oil becomes thinner and more fluid as it heats up, and thicker as it cools. Within reasonable limits, a thicker oil generally maintains a more robust lubricating film between moving parts and provides better sealing for critical engine components. With appropriate additives to help it resist excessive thinning in high heat, an oil can be rated for one viscosity when cold and a different viscosity when hot. The more resistant an oil is to thinning, the higher the second number will be (e.g., 10W-40 versus 10W-30), which is a desirable characteristic.
Conversely, in low temperatures, oil must resist excessive thickening to ensure proper flow to all moving parts within your engine. Excessive thickness can impede engine starting and reduce fuel economy. If the oil is too thick, the engine requires more energy to rotate the crankshaft, which is partially submerged in an oil bath. A lower number before the ‘W’ is preferable for cold-weather performance, thus a 5W oil is commonly recommended for winter use. However, synthetic oils can be engineered to flow even more easily when cold, enabling them to pass tests that achieve a 0W rating.
Once the engine is running, the oil heats up, which is why a higher second number becomes particularly important for extreme operating conditions and hotter-running, more complex engines.
The Abundance of Oil Types
Upon visiting auto parts stores, you’ll encounter oils specifically labeled for diverse applications: high-tech engines, new cars, higher-mileage vehicles, heavy-duty/off-road SUVs, and even vehicles from specific countries. You’ll also find a broad spectrum of viscosities.
Referring to your owner’s manual will inform you of the oil the vehicle manufacturer recommended for use when it was new. The manual might also mention “Energy Conserving” or “Resource Conserving” oils, indicating that the oil has passed a fuel economy laboratory test against a reference oil. While this doesn’t always translate directly to improved fuel economy, most leading brands offer at least some viscosities with this designation.
Choosing Between Synthetic and Conventional Motor Oil
- Premium Conventional Oil: This represents the standard oil for new cars. All major brands offer these oils, available in various viscosities and tested to the latest API service levels. Automakers typically specify a 5W-20 or 5W-30 oil for colder temperatures, with a 10W-30 oil as an optional choice for higher ambient temperatures. These three ratings encompass the majority of light-duty vehicles on the road. Even more crucially, consistent oil and filter changes are paramount. We suggest changing your oil every 4,000 miles or four months. The absolute minimum is twice per year. If your car features an electronic oil-change indicator on its instrument cluster, adhere to its guidance and remember to reset it after your oil change is complete.
- Full Synthetic Oil: Oils formulated for high-tech engines or demanding uses, whether for a Ford F-150 that frequently tows or a Chevrolet Corvette with the latest supercharged LS engine, are rich in synthetic additives. Their labels indicate whether they’ve successfully passed rigorous specialized tests for superior, long-lasting performance across all critical aspects, from viscosity index to protection against deposits. They flow more effectively at low temperatures and maintain optimal viscosity at high temperatures. So, why wouldn’t everyone use them? These oils are expensive, and not every engine requires them. There might even be specific features your engine needs that synthetic oils may not inherently possess. Again, always follow the recommendations in your owner’s manual.
- Synthetic Blend Oil: These oils combine a portion of synthetic oil with conventional (organic) oil, designed to offer enhanced protection for somewhat heavier engine loads and higher temperatures. Generally, they are less volatile, meaning they evaporate significantly less, which reduces oil loss and improves fuel economy. These oils are popular among drivers of pickups or SUVs who desire additional protection for activities that place more stress on the engine, such as hauling heavy loads. They are also considerably more affordable than full synthetics—sometimes only marginally more expensive than a premium conventional oil.
- Higher-Mileage Oil: Modern vehicles simply have longer lifespans. If you intend to own your car for a long time, pushing its mileage well into six figures, you have another oil option: oils specifically formulated for higher-mileage vehicles. Nearly two-thirds of vehicles on the road currently have over 75,000 miles on the odometer. Consequently, oil companies have recognized this area of consumer interest and are recommending new oils tailored for these vehicles.
When your vehicle is somewhat older and has accumulated substantially more mileage, you might notice occasional oil stains on your garage floor. Engine seals, such as those around the crankshaft, may have hardened and lost their flexibility, leading to leaks and potential cracking, particularly at lower temperatures. You’ll need to monitor your oil levels more frequently and may need to add oil between regular changes.
Higher-mileage oils are formulated with conditioners that permeate the pores of engine seals, restoring their shape and enhancing their flexibility. Most rubber seals are engineered to swell just enough to stop leaks, and oil refiners meticulously select their “reswelling” ingredients. Valvoline, for example, has shared performance data for one of their seal conditioners that caused most seal materials to swell while simultaneously reducing the swelling of a specific seal material that tended to over-expand due to ingredients found in some other engine oils.
You might also have observed some decline in performance and engine smoothness in your higher-mileage vehicle due to engine wear. These higher-mileage oils also tend to have somewhat higher viscosities. Even if the numbers on the container don’t explicitly indicate it, there’s a relatively wide range for each viscosity rating, and higher-mileage oils typically fall at the upper end of each range. They may also include additives to improve their viscosity index. The result? They provide better sealing for your pistons against their cylinder walls and will leak less through larger engine bearing clearances that have worn down over time. They might also contain a higher concentration of anti-wear additives to help slow down the wear process.
For an older vehicle, these features might offer greater benefits than what you’d derive from a full synthetic oil, often at a fraction of the cost.
Delving Deeper
An oil’s resistance to thinning at hotter temperatures is termed its viscosity index. While a higher second number is desirable, the oil also needs to be robust, maintaining its properties for thousands of miles until the next oil change. Oil tends to lose viscosity due to shear, which is the sliding motion occurring in the tight clearances between metal surfaces, such as those found in bearings. Therefore, resistance to viscosity loss—known as shear stability—is essential for the oil to maintain the lubricating film between these parts.
Unlike antifreeze, which is 95 percent composed of a single base chemical (typically ethylene glycol), petroleum-based engine oil contains a blend of several different types of base oils—some of which are more expensive than others. Oil companies typically select from five groups, each produced using a distinct method and yielding different viscosities. The more expensive groups undergo more extensive processing, in some cases through methods that result in a lubricant capable of being classified as a synthetic. So-called “full synthetics” contain chemicals that may be derived from petroleum but are so chemically altered that they are no longer considered natural oil. 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 constitutes anywhere from 70 to 95 percent of the mixture; the remainder is composed of additives. An oil with only 70 percent base oils isn’t necessarily superior to one with 95 percent base oils. Some base oils possess natural characteristics or properties derived from their processing that reduce or eliminate the need for certain additives. Although some additives enhance lubrication, they don’t necessarily possess great lubricity on their own.
The ingredients within an additive package vary in cost, but price is just one consideration. Some additives perform better in specific combinations of base oils. Similarly, some less expensive base oils are a good choice for a blend due to their performance with popular additives. In essence: every motor oil has a unique “recipe.” Refiners develop a set of objectives based on the needs of their customers (including the automakers themselves) and formulate oils to best achieve those goals.
Preventing oil from thinning as it heats up while enduring engine operation is one challenge, but it’s equally important to prevent oil from becoming excessively thick. Using less volatile premium base oils to inhibit evaporation is one approach. Evaporation of the base oil package not only increases oil consumption but also results in thicker oil, which decreases fuel economy.
Oil Additives
Oil companies’ use of additives represents another strategy for improving and maintaining oil performance. High engine temperatures combined with moisture, combustion byproducts (such as unburned gasoline), rust, corrosion, engine-wear particles, and oxygen can lead to the formation of sludge and varnish, which can clog and damage the engine. Additives help preserve effective lubrication by minimizing sludge and varnish. Here are the primary categories of additive ingredients and their significance:
- Viscosity-index improvers: These additives reduce the oil’s tendency to thin as temperature increases.
- Detergents: Unlike laundry detergents, detergents in oil do not scrub engine surfaces. While they do remove some deposits—primarily solids—their main purpose is to keep surfaces clean by inhibiting the formation of high-temperature deposits, rust, and corrosion.
- Dispersants: These additives disperse solid particles by keeping them suspended in the oil, preventing them from coalescing to form sludge, varnish, or acids. Some additives function as both detergents and dispersants.
- Anti-wear agents: Sometimes the lubricating film created by the oil breaks down, requiring anti-wear agents to protect the metal surfaces. A long-favored compound is ZDDP (zinc dialkyl dithiophosphate), a zinc and phosphorus compound, along with other phosphorus (and sulfur) compounds.
- Friction modifiers: These differ from anti-wear agents. Their role is to reduce engine friction, which can lead to improved fuel economy. Graphite, molybdenum, and other compounds are used for this purpose.
- Pour-point depressants: Just because a 0 degrees Fahrenheit viscosity rating is low doesn’t guarantee that the oil will flow easily at very low temperatures. Oil contains wax particles that can congeal and impede flow, so these additives are used to ensure it continues flowing in cold conditions.
- Antioxidants: With stricter emissions regulations leading to higher engine temperatures, antioxidants are essential to prevent oxidation, which causes the oil to thicken. Some additives that perform other functions also serve this purpose, such as anti-wear agents.
- Foam inhibitors: The crankshaft whipping through the oil in the oil pan can cause the oil to foam. Oil foam is less effective as a lubricant than a liquid stream, so oils contain foam inhibitors that cause these foam bubbles to collapse.
- Rust or corrosion inhibitors: These additives safeguard metal parts from acids and moisture.
More Is Not Always Better
You cannot necessarily enhance an oil’s performance by simply adding more additives. In fact, doing so can worsen its properties. For example, sulfur compounds possess anti-wear and anti-oxidation characteristics, but they can diminish fuel economy and reduce the effectiveness of your catalytic converter. Excessive amounts of a specific dispersant can adversely affect catalyst performance and decrease fuel economy. Anti-wear and friction-reducing additives may also contain ingredients, such as sulfur (which companies are pressured to reduce), that could impact catalyst performance. Adding too much of certain detergents can also negatively affect anti-wear characteristics.
Don’t Overlook the Filter
Oil filters are a distinct, though related, topic when it comes to changing your oil. Again, it is always best to consult your owner’s manual for the specific type of filter required. Some aftermarket filters are larger, so ensure you have extra oil if you opt for one.