Physical characteristics:

• Density – one of the fundamental physical properties;
• Color – the simplest indicator that allows to make first assumptions about the oil quality;
• Viscosity – determines the lubricant's film strength and efficiency in preventing friction between moving parts;
• Viscosity index – allows for a better comparison of the viscosity behavior of different oils based on temperature;
• Content of mechanical impurities – can lead to system malfunction in critical applications;
• Flash point – since it is an important constant for all types of lubricants, each oil is tested based on the flash temperature.

Physicochemical characteristics:

• Water content – when water is present, the lubricant’s viscosity changes considerably and chemical changes also occur and result in the formation of varnish, sludge, and acids;
• Acid and base number – help to monitor the effective life of in-service lubricants;
• Determination of elements by inductively coupled plasma atomic emission spectrometry (ICP-AES) – the appearance of wear metals in lubricating oils is a significant indicator of contamination;
• Water separability – the ability of lubricating oil to separate water and resist emulsion is an important performance characteristic for applications involving water contamination and turbulence;
• Determination of polycyclic aromatics – these compounds arise from incomplete combustion during engine operation, escape the combustion chamber, leading to lubricant contamination;
• Corrosive effects – a lubricating oil may become unsuitable for its intended use as a result of one or several processes, notably oxidation, water and air contamination. Oxidation can lead to such undesirable consequences as acidity and corrosion;