A mineral or synthetic designation refers to the base oil that makes up about 75 to 80 percent of each bottle of finished fluid. The other 20 to 25 percent of the bottle is additives like detergents, anti-foaming agents, buffers, viscosity index improvers, and sacrificial lubricants that provide a last-ditch barrier to prevent metal-to-metal contact. Nearly all additives are synthesized in a lab, but these additives aren’t considered when categorizing oil. It’s the base oil that makes the difference, and the American Petroleum Institute (API) has split them up into five groups based on key characteristics like sulfur content, saturate content, and viscosity index (VI). The VI isn’t the number that’s printed on the bottle but rather an arbitrary measure of the base oil’s viscosity change over a range of temperatures. The VI is closely related to the oil’s ability to reduce friction, independent of additives.