ESDU 94020 primarily applies when there is a continuous lubricant film as in hydrodynamic or hydrostatic bearings, but also has application in boundary lubrication. It does not treat greases. Classes of lubricants considered are mineral oils (for comparison), synthetic hydrocarbons, diesters, polyol esters, polyglycols, phosphate esters, silicones, silicate esters and disiloxanes, polyphenyl- and C-ethers, and perfluoropolyalkyl ethers (PFPAE). Graphs illustrate the comparative behaviour of them in terms of working temperature based on viscosity limits, pressure exponent of viscosity, thermal stability, oxidation limits, resistance to radiation, flash point, ignition temperature, and relative cost. A table also compares their density, thermal conductivity, thermal capacity, bulk modulus, toxicity, effect on environment, and biodegradability. A data sheet is given for each class of lubricant giving broad guidance on viscosity/temperature behaviour, volatility, fire resistance, boundary lubrication characteristics, and a table of advantages and disadvantages. The properties of typical members of each class are indicated.