The API symbol, often called the "donut," provides detailed information about engine oil, including its performance category, SAE viscosity grade, and other relevant specifications. It serves as a quality mark indicating that the oil meets certain performance standards set by vehicle and engine manufacturers as well as the lubricant industry. The donut also shows gas and diesel engine oil service categories, resource conservation status, and other certification details that help consumers select oils suitable for their engines. This symbol is used by marketers as a warranty that the licensed oils comply with the requirements of API standards. It displays key information in specific areas of the symbol, such as the Service Categories in the upper portion and resource conserving classifications in the lower part. This standardized symbol helps consumers easily identify licensed and certified engine oils for gasoline and diesel vehicles.

For contrast, there is also a Donut API that provides up-to-date information on donuts from Krispy Kreme's website, but this is unrelated to the API service symbol or donut in the context of engine oils.

In summary, the API symbol or "donut" provides information about:

• Engine oil performance categories (gasoline and diesel)
• SAE viscosity grade
• Resource conserving oil classifications
• Certification and licensing status under API standards