Every DIYer has stood in the motor oil aisle of an auto parts or big-box store, eyes glazed over from the bewildering array of engine oils before you. But choosing the right one for your car’s engine doesn’t have to be a confusing experience.
Armed with the guidance we’re providing below, you can cut through the clutter of endless choices and find exactly the right oil for your car’s engine. From viscosity grades to service classifications to high-mileage oils, we’ve got you covered.
What Oil Grade Numbers Mean
One of the most important aspects of engine oil is its viscosity. Think of viscosity as the oil’s thickness or the resistance to flow. Oil viscosity has a significant influence on engine wear, fuel economy and performance. In terms of viscosity, you can think of oil as like honey in the way it thickens when cold and thins when hot.
Engine oil was originally produced in fixed viscosity grades specified by the Society of Automotive Engineers (SAE). The nomenclature is pretty transparent. Higher numbers denote thicker oil at a given temperature. SAE 40 is thicker than SAE 30, and so on. Each grade allows for a specific range of viscosity, where an increase of 10 on this scale represents roughly 25% thicker oil. You’ll often hear these oils referred to as “straight-grade” or “straight-weight” oils, even though weight has nothing whatsoever to do with oil grades.
Far more common today are multi-grade oils, which you can spot by the W and hyphen in their identifier (0W-20, 5W-40, etc.). The big advantage of multi-grade oil is improved flowability at low temperature, which allows the oil to reach the engine’s internal bits more rapidly during cold starts, reducing wear. Cold starts are when the majority of engine wear occurs.
With that background, let’s deconstruct a common multi-viscosity oil grade. For 5W-30 grade oil, the “5W” refers to its grade at low temperature (the “W” stands for “Winter”) and the “30” refers to its grade at higher temperature. Again, lower numbers denote lower viscosity and it’s best to use oil that has the viscosity recommended by the manufacturer or engine builder.
The Next Important Consideration
Next up, is the oil’s API service classification. API stands for American Petroleum Institute, which is the independent organization that performs the testing and sets these standards for the petroleum industry. On the back of every bottle of oil there’s a symbol and a code that denotes which classification the oil meets.
The API Service Classification codes define the performance standards of oil properties (other than viscosity) that meet the demands of newer and newer engines. As engines evolved through the years, oil formulations needed to keep pace in areas like seal compatibility, sludge and varnish control, resistance to forming deposits, low-speed pre-ignition abatement, etc.
Later service classifications denote a more capable oil. So, while you can safely run an older engine on a more recent API service classification, it’s not a good idea to run a newer engine on an older API service classification because the oil is not as tolerant of the heat generated by more modern engines. Heat breaks down the oil, leading to accelerated engine wear.
Currently, active service classifications for gasoline engines are SJ, SL, SM, SN and SP (the first letter is always an “S,” and the second letter is changed in alphabetical order when API releases a new standard.). API has been at it for a while—the original SA standard governed the oil used in vehicles built prior to 1930! Check api.org for a breakdown of the classifications.
What Oil Grade and API Service Classification Does My Car Need?
These standards are admittedly pretty convoluted, and you’ll be forgiven if it’s all a bit daunting. The good news is that your owner’s manual is the cheat code to tell you exactly which oil grade and API Service Classification your car’s engine requires.
So, if your owner’s manual tells you to run an oil with API Service Classification SM or newer, you can safely use SM, SN or SP. Just don’t run an older spec like SJ. Older spec oils aren’t widely available, but it never hurts to check the label.
Some automakers even go a step further and call for their own unique specifications, like GM’s Dexos. Mercedes-Benz even has its own four-digit specification. These will be called out in the owner’s manual, too. Fortunately, all you need to do is look for oil that lists the required approval on the bottle.
What’s the Deal With Conventional and Synthetic Oil?
Engine oils are composed of two broad categories of ingredients: the base stock and the additive package. We’re simplifying here because engine oil is a highly complex animal, but the base stock is essentially the lubricant itself, and the additive package consists of compounds that are dissolved into the base stock to influence various characteristics of the oil.
The benefit of synthetic base stocks is that they are inherently better able to cope with high temperatures and do not oxidize as readily as do conventional oils. They don’t lean as heavily on the additive package, which is generally what gets depleted first as an oil accumulates use. This means synthetic oils are typically longer-lasting and do not need to be changed as often.
Synthetic oils are also considerably more expensive. While a quart of conventional oil can cost as little a few bucks, synthetic oils usually cost two to four times more.
Then there’s Synthetic-Blend oil, which is exactly that; oil that consists of a mixture of conventional and synthetic base stocks. This category of oil splits the difference between conventional and full-synthetic oil in performance and cost.
What About “High Mileage” Oil?
“High Mileage” oils are tailored to address issues common to engines that are getting on in miles. Usually, their labels call out around 75,000 miles.
These oils are formulated with an additive package that has conditioners to help older rubber seals work better and specific detergents to remove deposits. The idea is to help prevent aging gaskets from drying out and causing oil leaks, and to reduce the amount of unwanted contaminants that might already be stuck onto your engine’s piston rings.
Otherwise, high-mileage oils are not drastically different from their non-high-mileage counterparts. And there’s no harm in running these oils in a low-mileage engine.
Should I Run Higher Viscosity Oil in My Car?
There’s a school of thought that thicker oils are better for high-performance engines. This isn’t necessarily true. Running a thicker oil than your engine was designed for has downsides.
For one, thicker oil can rob power by increasing drag on the moving parts inside the engine. Try vigorously stirring honey that was stored in the fridge. Now do the same thing to honey that has been warmed up on the stove. That difference in effort is what your engine experiences when running on thicker oil. This is why racing oils are often thinner, to minimize drag and maximize engine power. But remember, most car races only last a few hours, not thousands of miles!
Also, consider that the engine’s piston rings may not have sufficient tension to effectively scrape the thicker oil from the cylinder walls. If this happens, the resulting poorer ring seal allows more oil into the combustion chamber, reducing octane and making the engine more knock-prone, which can cause engine damage. It also increases blow-by, where combustion gases enter the crankcase. Ironically, the fuel content of the additional blow-by will tend to dilute the oil, decreasing its viscosity and shortening the oil’s life. That’s why it’s best to follow the manufacturer’s recommendations.
Final Thoughts
Choosing the right oil for your car’s engine is one of the most important choices a DIYer can make for the long-term health of your vehicle. So, the next time you’re gearing up to give your car an oil change, choose your motor oil carefully. Don’t just go with the cheapest stuff. Now you have the information you need to make an educated choice.
Whether you ultimately choose conventional or synthetic, synthetic blend or high-mileage, it’s important to always remember that the right oil for your car’s engine should align with your vehicle manufacturer's specifications, the age (mileage) of your vehicle and the conditions you drive the most.
