Full definition
Fluid that reduces friction and wear between metallic surfaces in motion through hydrodynamic or elastohydrodynamic film. Base types: mineral (petroleum refining), synthetic PAO (polyalphaolefins), ester, polyglycol. ISO VG defines viscosity: 32 for hydraulic, 68 for light gears, 220-320 for industrial gearboxes, 460+ for low-speed reducers. Brands: Mobil DTE, Shell Omala, Castrol Optigear, Fuchs Renolin. Standard: ISO 6743.