ISO Grade or, strictly, ISO Viscosity Grade, is a system of classification for liquid lubricants--principally industrial oils--developed by the International Standards Organization and other bodies in the 1970s.
The purpose of the ISO Grade was to establish a universal classification for liquid lubricants that can be used, without confusion, by suppliers, designers and users.
ISO Grade is based on viscosity--the resistance to flow, or “thickness,” of a liquid--and each successive grade is approximately 50 percent more viscous than the previous grade. An ISO Grade 32 oil, for example, has a viscosity of between 28.8 and 35.5 centistokes (cSt) at 40 degrees Centigrade (104 degrees Fahrenheit), giving an average of 32cSt.
ISO Grade and other, similar classification systems are important because low viscosity oil may to too thin to adequately protect components and high viscosity oil may result in sudden increases and decreases in oil pressure (“cavitation”) that cause mechanical damage.
