Navigating the Motor Oil Aisle
With so many motor oil options available, selecting the right one for your car can feel like a complex task. But the initial step is surprisingly simple:
The owner’s manual for your car is the ultimate guide. It lists the recommended oil weight, which will be a standard format like 10W-30 or something a bit less common. This number refers to the viscosity of the oil, or its thickness. You’ll need to adjust which weight and type of oil you use based on the seasons and how you typically use your vehicle, which we’ll explain further below. When using your car in moderate temperatures, what your owner’s manual specifies is fine. It’s also important to always choose an oil from a brand that clearly displays the starburst symbol, which affirms that the oil has been tested by the American Petroleum Institute (API).
Additionally, you’ll see 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 letters are based on a series of laboratory and engine tests that determine the oil’s capability to protect the engine from wear, high-temperature deposits, and sludge. API provides a comprehensive list of these standards. Make sure the oil you purchase has gone through testing under a recent and relevant standard. At the time 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. These are the basics to get you started.
Understanding the Fundamentals
Viscosity
Viscosity is a fluid’s resistance to flow, and the viscosity of most motor oils is rated based on how thick it is at zero degrees Fahrenheit (the number preceding the “W” which stands for winter), as well as its thickness at 212 degrees (the second number after the dash).
Motor oil becomes thinner and runnier as it gets heated up and thickens when it cools. Generally, a thicker oil maintains a better film of lubrication between moving parts and seals the crucial components of your engine better. The right additives can help the oil resist thinning too much in the heat, allowing an oil to be rated for one viscosity when cold and a different viscosity level when it’s hot. The more resistant oil is to thinning, the higher the second number (like in 10W-40 versus 10W-30).
Low temperatures require oil to be resistant to excessive thickening to ensure it can still flow correctly to all the moving parts of the engine. Excessive thickness makes it harder to start the engine, which reduces overall fuel economy. When the oil is too thick, the engine needs more energy to turn the engine’s crankshaft, which is partly submerged in oil. A lower number before the W is better cold-weather performance, so a 5W oil is usually what’s recommended for winter use. However, synthetic oils can be formulated to flow even better when cold, so they can 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 or more complicated, hotter-running engines.
Why So Many Oils?
In auto parts stores, you’ll find oils labeled for specific purposes, such as high-tech engines, higher-mileage vehicles, new cars, and heavy-duty or off-road SUVs. There are many varieties of viscosities. When you read your owner’s manual, you’ll know what oil the vehicle manufacturer recommended when the car was brand-new. The manual may reference Energy Conserving or Resource Conserving oils. If your car runs on these types of oils, it means the oil has passed a fuel economy lab test against a reference oil. While this doesn’t always translate to better fuel economy, most leading brands have at least some viscosities that are labeled this way.
How to Choose Between Synthetic and Conventional Motor Oil
Here’s a breakdown:
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Premium Conventional Oil: This is the standard new-car oil. All leading brands have these oils, which come in several viscosities and have been tested under the latest API service level. Automakers often put on the specifications sheet a 5W-20 or 5W-30 oil for colder temperatures, with something like a 10W-30 oil as optional for higher ambient temperatures. These three ratings cover most of the light-duty vehicles on the road. More importantly though, you need to change the oil and filter on a regular schedule. We recommend changing your oil every 4,000 miles or four months. The absolute minimum is every six months. If your car has an electronic oil-change indicator, follow its guidance and make sure to reset it after your oil change.
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Full Synthetic Oil: Oils made for high-tech engines or heavy-duty uses, whether it is a Ford F-150 that tows frequently or a Chevrolet Corvette with the latest supercharged LS engine, use full synthetic additives. These oils’ labels show whether they have passed stringent special tests for exceptional, and longer-lasting performance in all critical areas, from the viscosity index to protection against deposits. They flow better at low temperatures and maintain peak viscosity in high temperatures. They can be more expensive though, and not every engine requires them. Follow the guidance in your owner’s manual.
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Synthetic Blend Oil: These types of oil mix a dose of synthetic oil with organic oil and are formulated to provide protection in heavier engine loads and high temperatures. This generally means they’re less volatile, which results in less evaporation, and in turn, reduced oil loss that can lead to increased fuel economy. These oils are popular with pickup or SUV drivers to add an extra layer of protection for activities that put more stress on the engine, like hauling heavy loads. They are also far less expensive than full synthetics—sometimes only slightly more than a premium conventional oil.
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Higher-Mileage Oil: Today’s vehicles are known to last longer than ever before. If you prefer to pay your car off and keep the mileage well into the six figures, you have the option of using oils specially formulated for higher-mileage vehicles. Almost two-thirds of the vehicles on the road have over 75,000 miles on the odometer. Oil companies took note, and as a result, they now have new oils they recommend for these vehicles.
When your vehicle is somewhat older with considerably more accrued mileage, you might notice a few oil stains on the garage floor. Engine seals, such as those around the crankshaft, may become hardened and lose some flexibility, leading to cracks and leaks, especially in colder temperatures. You’ll need to check oil levels more often, and you may need to top off the oil between oil changes. Higher-mileage oils include conditioners that flow into the pores of engine seals to restore their shape and increase flexibility. Most rubber seals are designed to swell just enough to stop leaks, so oil refiners need to be careful picking their “reswelling” ingredients.
Higher-mileage oils also have higher viscosities. Even if the numbers on the container don’t indicate it, there’s a fairly wide range for each viscosity and the higher-mileage oils are at the top of each respective range. They also may include additives intended to improve their viscosity index. This helps seal your pistons more effectively against cylinder walls to prevent oil leaks. They can even improve how the engine works where engine bearing clearances have worn down over time. They may also have a higher dose of antiwear additives to slow that wear process.
Going deeper:
An oil’s resistance to thinning in hotter temperatures is called the viscosity index. Although a higher second number is good, the oil also needs to be robust and last for thousands of miles before the next oil change. Oil tends to lose viscosity from shear, which is the sliding motion in the tight clearances between metal surfaces (bearings in particular). So, resistance to viscosity loss, typically called shear stability, is necessary to enable the oil to maintain the lubricating film between engine components.
Unlike antifreeze, which mostly consists of a single base chemical, petroleum-based engine oil contains a blend of various base oils. Oil companies typically work from a selection of five groups, each produced differently and in varying viscosities. The most expensive groups are more highly processed, sometimes with methods that produce a lubricant that can be classified as synthetic. Full synthetics contain chemicals that might come from petroleum but are altered to the point that it’s no longer considered natural oil. For example, one custom blend involved 10% polyalphaolefins (PAO), the most common type of chemical used in full synthetic oil.
The base oil package in any oil makes up about 70% to 95% of the mix; the rest comes from additives. An oil with 70% base oils isn’t necessarily better than one with 95% base oils. Some base oils have characteristics that reduce or eliminate the need for additives.
The Role of Additives
The ingredients in an additive package vary in cost, but price is just one factor. Some additives work better in certain combinations of base oils. Refiners come up with a list of objectives based on the needs of their customers, including the carmakers themselves, and they formulate oils accordingly to meet those goals as best as they can.
Motor oil additives help to maintain good lubrication. Here are the key categories of additive ingredients and why they’re important:
- Viscosity-index improvers: These reduce the oil’s tendency to thin with increasing temperature.
- Detergents: These remove some deposits, primarily solids. Their main goal is to keep surfaces clean by preventing high-temperature deposits, rust, and corrosion.
- Dispersants: These disperse solid particles by keeping them in a solution, and therefore by preventing them from coming together to form sludge, varnish, or acids. Some additives function as both detergents and dispersants.
- Antiwear agents: These protect metal surfaces. A zinc and phosphorus compound called ZDDP is a classic favorite, along with other phosphorus (and sulphur) compounds. ZDDP stands for zinc dialkyl dithiophosphate.
- Friction modifiers: They reduce engine friction and, therefore, can help overall fuel economy. Graphite, molybdenum, and other compounds are used for this.
- Pour-point depressants: Just because a 0-degree Fahrenheit viscosity is low doesn’t mean that oil will flow readily at low temperatures. Oil includes wax particles that can congeal, so these additives are used to help the oil flow more easily when it’s cold.
- Anti-oxidants: With tighter emissions regulations leading to higher engine temperatures, antioxidants prevent oxidation that thickens the oil. Some multi-function additives also serve this purpose, such as the antiwear agents. If you aren’t familiar, oxidation is a chemical process that results in the loss of electrons, or the addition of oxygen or some other element. It involves a transfer of electrons between substances.
- Foam inhibitors: The crankshaft whipping through oil in the oil pan causes oil to foam. This foam is not as effective a lubricant as a liquid stream, so foam inhibitors can prevent the bubbles from forming. There are also other additives out there which can create the reverse of foam to try and lubricate more effectively.
- Rust or corrosion inhibitors: These protect metal parts from acids and moisture.
Don’t Forget the Filter
Oil filters are a completely separate but related subject when it comes to changing your oil. Again, it’s best to check your owner’s manual to determine the appropriate filter. Some aftermarket filters are larger, meaning if you use one, it’s best to have a little extra oil on hand.
