Transmission oil, also known as automatic transmission fluid (ATF), is a specialized lubricant engineered for use within the sealed environment of automatic and some manual transmissions. Its primary function is to transmit hydraulic pressure to engage gears and clutches, while simultaneously lubricating moving parts to minimize wear. Modern formulations incorporate viscosity index improvers, friction modifiers, and detergents to maintain consistent performance across operating temperatures and prevent deposit formation. Specific ATF types are designated by manufacturer standards (e.g., Dexron, Mercon, ATF+4) dictating compatibility and performance characteristics. Synthetic and mineral oil-based formulations are available, varying in thermal stability and service life.