Lubricants age over the course of their operating time. Oxidation, nitration and sulfation of an oil are typical indicators for this. They indicate how far the aging process of a used oil has progressed. Oxidation is usually used as a synonym for classic oil aging. Nitration and sulfation indicate the contamination of oils from combustion engines with nitrogen oxides (NOx) and sulfur compounds (SOx), respectively.

Due to oxidation, nitration and sulfation, the oil becomes "acidic" and can cause corrosion. An increase in viscosity is often observed as well. Oxidation, nitration and sulfation are determined by FT-IR spectroscopy by comparing a used oil spectrum with a fresh oil. The unit for these parameters is A/cm.  "A" stands for absorption of infrared light at the corresponding wavenumber. "cm" refers to the layer thickness of the oil where the light goes through.