• Conventional: Refined from crude oil and commonly recommended for older engines with simpler designs that are not expected to endure high-performance demands while driving. Conventional oil is typically less expensive, but generally requires more frequent changes.
• Semi-Synthetic: A blend of conventional and synthetic oils (see below) that provides enhanced performance at a lower price point than a full-synthetic oil. Most late-model cars require semi-synthetic oil to meet automakers’ specifications.
• Synthetic: Oils that are chemically engineered at the molecular level to reduce impurities, flow more easily at low temperatures and resist breakdown at high temperatures. Synthetic oils are more expensive, but offer the highest level of protection. The engines in many premium luxury cars and high-performance models require the use of synthetic oil.
• High-Mileage: Designed for engines with 75,000-plus miles of use, high-mileage oils contain additives that help limit common problems such as oil leaks and increased oil consumption.
• Society of Automotive Engineers (SAE) Viscosity Grade: The SAE viscosity grade—in the center section of the donut icon—refers to an oil’s weight or thickness, and indicates how easily it will flow to lubricate moving engine parts. Because outside temperature impacts viscosity, most modern vehicles call for multi-viscosity oils that perform well in a wide range of temperatures throughout the year.
• American Petroleum Institute (API) Service Category: This code appears in the top half of the donut symbol and designates which API standard(s) the oil meets. The testing required to meet API standards involves a wide variety of requirements, from engine protection and performance to emissions control and compliance with government regulations.
Each category consists of a two-letter code, beginning with “S” for gasoline (spark) engines or “C” for diesel (compression) engines. The second letter denotes how current a standard the product meets, with “SN” and “CK-4” representing the most current standards as of January 2018.
In gasoline engines, you can use a newer service category of oil than that required by the vehicle manufacturer —for example, “SN” can be used in engines that call for “SM”. But, it is never okay to use oil with an older service category than that specified. The situation is more complicated with diesel engines, where the latest oil service category is not always compatible with earlier engine designs. Always follow the automaker’s recommendations for these engines.
• International Lubrication Standard and Approval Committee (ILSAC) Standard: This rating for oil performance and engine protection is established by ILSAC, a standards organization made up of American and Japanese automakers. “GF-5” is the most current standard as of January 2018, and the presence of the starburst symbol on an oil package indicates compliance with the standard’s requirements for emissions control, seal compatibility, protection of turbochargers and pistons against high-temperature deposit formation and more.
• Automaker and Diesel Engine Manufacturer Standards: In addition to the industry standards described above, most automakers and diesel engine manufacturers have proprietary motor oil standards designed around the unique needs of their powerplants. The standards are identified by alpha-numeric codes that appear in owners’ manual and on the packaging of oils that meet the necessary requirements.