Motor oil is critical for getting the longest service life from your car’s engine: It lubricates the engine’s internal parts, minimizes wear, disperses heat, and helps prevent corrosion and sludge buildup. After 40 hours of research and interviewing numerous experts, we’ve found that there’s little difference between similar motor oil from the major brands, but we recommend Amsoil full-synthetic engine oil because it’s highly regarded by every expert we talked to, it’s chosen as the factory-fill oil by several major automakers for their premium and high-performance cars, it’s easy to find in retail outlets, and it’s available in a wide selection of formulas for both newer and older vehicles. In other words, it’s as safe a bet as you can make.
Though conventional motor oil works well for many drivers (as long as it’s changed on schedule), a good synthetic will protect your engine in a greater range of conditions and will hold up better over time. Like all full synthetics, Amsoil costs more than conventional oil, but we think the added protection is worth it.
More important than which brand of oil you buy is that you choose the right one for your vehicle by looking for industry certification and the right viscosity, and determining whether you want a conventional or synthetic formula (or a blend of the two)—we talk about that below. And even the greatest oil in the world eventually picks up contaminants and breaks down from heat and friction, so you should regularly check its level and change it on schedule to prevent serious damage and to avoid voiding your car’s warranty.
What to look for when buying motor oil
The array of oil varieties looks bewildering at first, but breaking the decision down into steps makes it a lot more manageable.
Check for certification logos
Any motor oil you buy should have the American Petroleum Institute’s star logo. All motor oils sold in the US are tested by the API, and if the oil meets certain minimum qualifications, the manufacturer can place the institute’s logo on the package. Also check the API “donut” that’s shown elsewhere on the bottle: The designation “API Service SN” indicates that it meets the latest API oil category specification for today’s gasoline engines. The designations SM, SL, and SJ are used for older vehicles. Oils with other designations, such as SH, SG, and SF, are considered obsolete and not for use in most vehicles built in the past 20 years.
Look for the API star logo, which shows the oil met the industry’s minimum performance standards. To earn GM’s Dexos certification, an oil must meet more stringent standards.
As long as the oil’s packaging carries those two logos, you can be sure the oil meets the API’s minimum performance requirements, even if it’s from an inexpensive off-brand. However, some of the mechanics we spoke to are wary of generic oils. According to Kevin Knox, an auto technician at Mullane Motors in Lockport, New York, with 30 years of experience, “If you see some generic oil at one of the big box stores, even if it meets the SAE [Society of Automotive Engineers, which conducts oil viscosity tests] rating, it might not be as good, it might not have the same lubricating qualities and resistance to heat and pressure. It’s not worth saving 50 cents or 75 cents a quart.”
As long as the oil’s packaging carries the API star logo and the API “donut,” you can be sure the oil meets the API’s minimum performance requirements.
General Motors’s Dexos certification sets an even higher bar than the API testing, requiring an oil to meet a more stringent set of conditions. However, GM requires oil manufacturers to pay a licensing fee to carry the Dexos logo, so not all manufacturers include it on their packaging, even if their oil meets GM’s standards.
Because motor oil thins as it heats up, modern multigrade formulas show their viscosity rating during a cold start (in this example, the 5 next to W, for “winter”) and at high operating temperatures (the 30). Use only the type recommended for your car.
Get the recommended viscosity
Before buying, you need to know what viscosity is appropriate for your car. The “5W-30” (or similar) designation listed on the oil bottle refers to the oil’s viscosity properties during a cold start (the number next to the W, which stands for “winter”) and at high operating temperatures (the number without the letter); motor oil thins as it heats up. Viscosity grades for automotive motor oils, as established by SAE International, range from 0 to 60. Luckily, you don’t need to understand how those numbers affect an oil’s performance. Just buy whatever your owner’s manual says is required for your particular engine. If you’ve misplaced your owner’s manual, you can probably find the manual online, or use Mobil’s tool for seeing what viscosity is recommended for your car.
Choose between conventional oil or synthetic
Your car’s owner’s manual or the Amsoil tool can also help you decide whether to buy conventional or synthetic oil—some cars require synthetic oil, or require an oil viscosity available only in synthetic oils (such as 0W-20).
Motor oil experts agree that synthetics offer several advantages over conventional oil.
What’s the difference? Conventional oil is made from crude petroleum that comes out of the ground and is refined until it has the appropriate qualities. Synthetic oil is chemically developed from petrochemicals to have the exact molecular qualities the manufacturer wants. (The increased complexity of the latter process accounts for much of the higher cost of synthetics.) Both types are blended with various additives, such as detergents, viscosity index improvers, and rust inhibitors, to achieve certain characteristics.
Conventional motor oil is fine for most driving conditions, but motor oil experts agree that synthetics offer several advantages. First, synthetics have superior thermal stability, which means that the oil holds up better in extreme temperatures. For example, it better resists thickening at very low temperatures, so it flows through the engine more easily when starting the car on a cold morning. (The period just after starting typically causes the highest rate of engine wear.) Marlan Davis of Car Craft points out that a synthetic oil will still flow at temperatures more than 20 degrees Fahrenheit colder than the point where a conventional oil becomes too cold to lubricate the engine at startup. And a synthetic oil also holds up better in high-heat conditions, such as when towing a trailer or hauling heavy loads, when running an engine hard during hot summer months, or in a turbocharged or supercharged engine. Those are conditions in which a conventional oil more easily breaks down. Overall, the more extreme the conditions that the engine runs under, or the tighter the tolerances on that engine, the more a synthetic oil is going to shine.
As long as your car doesn’t meet any of the criteria mentioned above for using synthetic oil, you can safely use conventional oil. But because it’s chemically developed, synthetic oil also has fewer impurities than conventional oil, which helps it resist the formation of sludge and deposits inside the engine. And synthetics simply hold up longer in the engine, providing a longer service life.
In talking about the advantages of synthetic oil, Road & Track says,
When engineers design the engine, they’re looking to achieve a certain oil flow rate throughout the system, dependent on the temperature of the engine and the speed at which it’s rotating. As oils age, their flow characteristics change, and this changes how well your engine is protected from wear when it operates outside the boundaries of its initial design. While it’s true that from a viscosity standpoint alone, you could match the protection of a synthetic oil simply by changing your conventional oil regularly, often times synthetic oils come with superior quality additives, leading to a cleaner, smoother running engine as well. There’s a reason why most automotive manufacturers have switched over to synthetics directly from the factory.
That said, you should still change your oil on schedule—a synthetic oil just gives you a larger margin for error.
Full synthetics can cost a lot however — three times as much as conventional oils. In fact, some are even more expensive than that. Although some higher-priced synthetic oils—such as Royal Purple, Motul, or other specialty-branded oils are often highly touted on automotive forums, we’ve yet to see anything other than anecdotal evidence that these oils actually perform significantly better than other synthetics.