Classification of engine oils falls under the responsibility of the ____________.

The classification of engine oils is primarily the responsibility of the American Petroleum Institute (API). The API establishes a system for categorizing engine oils based on their performance and uses, which helps ensure that consumers can select the right oil for their specific engine requirements. This classification system includes designations like "API SN" or "API SM" that reflect various performance standards, including protection against wear, oxidation, and sludge, as well as the oil's compatibility with modern engines.

When it comes to other organizations mentioned, while the Society of Automotive Engineers (SAE) does play a role in defining viscosity grading and establishing testing standards, it is primarily the API that oversees the performance classification of engine oils. The Environmental Protection Agency (EPA) focuses on environmental regulations related to automotive emissions, while the Internal Revenue Service (IRS) is concerned with tax issues and has no role in the classification of engine oils. Therefore, the API is the recognized authority for engine oil classification, ensuring that the products meet industry standards and provide adequate protection for engine performance.