Density is understood to be mass per volume ratio. For oils, it is usually expressed in kg/m3 at the temperature of +15°C or +20°C. The density of lubricant oils varies from 700 to 950 kg/m3 depending on the base oil quality, thickness and additives used.
The thicker the liquid, the higher its viscosity. The viscosity of lubricant oils is usually measured in cSt (centistokes)= mm2/s (SI system) or cP (centipoise) = mPa·s (SI system). The temperature must be always specified with any unit of viscosity. Viscosity of all oils decreases strongly with increasing temperature. The viscosity of standard engine oil SAE 10W may be 2,000 cP at -20°C, but after heating up to +100°C the viscosity falls down to 5.2 cSt.
The viscosity index (VI) characterizes the ability of a liquid to maintain its viscosity over a wide temperature range. The more the liquid in question thins, the lower its viscosity index. VI of single-grade engine oils is approx. 95–110, while that of multi-grade oils may exceed 200.
Flash point reflects the extent of flammability of fluids. Flash point is the temperature at which the fluid emits so much flammable vapors that they flare up when lit with open fire while the fluid itself does not remain burning.
Ignition temperature is the temperature at which the gases evaporating from a fluid heated in an open fire pot burn for at least five seconds when lit with open fire. The ignition temperature is typically 10–50°C higher than the flash point.
Oil thickens with decreasing temperature. At a certain temperature it no longer flows at its own weight. This temperature is referred to as the pour point. The pour point depends, among other things, on the viscosity of the oil and its chemical structure. In paraffinic oils, thickening is caused by the wax contained in the oil, which forms crystals. The more the oil cools down the larger the crystals grow, eventually forming a network obstructing the flow within the oil.
When the engine is running, acidic compounds caused by the combustion of fuel enter the oil. They must be neutralized to prevent corrosion of metal parts. For this reason, engine oils contain additives to create an alkali reserve. Its amount is expressed in terms of total base number (TBN).
It is the minimum working temperature of transmission oils at which the oil stays fluid.