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.