###### Abstract  
Viscosity is considered to be a key factor in the quality of lubrication by oil and engine manufacturers and is therefore one of the most monitored parameters of lubricants. FTIR (Fourier-transform infrared) spectrometry in combination with Partial Least Squares (PLS) and Principal Component Regression (PCR) was therefore proposed and tested as an alternative to the standardized method for determining the kinematic viscosity at 100 \({}^{\circ}\)C with an Ubbelohde capillary viscometer (CSN EN ISO 3104) of worn-out motor oil grade SAE 15W-40. The FTIR-PLS model in the spectral region of 1750-650 cm\({}^{-1}\) with modification of the spectra by the second derivative proved to be the most suitable. A significant dependence of R = 0.95 was achieved between the viscosity values of 190 samples of worn-out motor oils, which were determined by a standardized laboratory method, and the values predicted by the FTIR-PLS model. The Root Mean Square Error of Calibration (RMSEC) parameter reached 0.148 mm\({}^{2}\)s\({}^{-1}\) and the Root Mean Square Error of Prediction (RMSEP) parameter reached 0.190 mm\({}^{2}\)s\({}^{-1}\). The proposed method for determining the kinematic viscosity at 100 \({}^{\circ}\)C by the FTIR-PLS model is faster compared to the determination according to the CSN EN ISO 3104 standard, requires a smaller amount of oil sample for analysis and produces less waste chemicals. ol 2021 11 3842. [[https://doi.org/10.3390/app11093842](https://doi.org/10.3390/app11093842)]([https://doi.org/10.3390/app11093842](https://doi.org/10.3390/app11093842))  
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M.Kucera, M.Kucera, I.V.V.C. new oil is usually allowed to ensure lubrication. The quantity that allows to evaluate the change in the viscosity of the lubricant depending on the change in temperature is the viscosity index. The higher the viscosity index value, the less the viscosity changes with temperature. Therefore, large viscosity changes can occur during motor oil operation. Products of thermo-oxidative degradation and soot in oil have an effect on the increase in viscosity. Oxidation of the oil is accompanied by its darkening, increase in its acidity and viscosity, odour and discharge of insoluble oxidation products [13]. The increase in viscosity can also be caused by the penetration of coolant into the oil due to an operational accident of the engine. The increase in motor oil viscosity also occurs when the base oil has a high evaporation rate, so that more volatile fractions decrease during operation. This is reflected in increased oil consumption and the unfavourable composition of exhaust emissions [14; 15]. On the contrary, the decrease in the viscosity of the operated oil is due to the dilution of the oil with fuel, the depolymerization of polymer additive used as a viscosity index modifier, or also due to cracking of the oil under thermal stress [16]. Soot is formed in a significant amount in oil cracking, contributing to increasing the viscosity of the motor oil. Exhaust after-treatment and regeneration systems can contribute to increased fuel supply to the motor oil. As many as tens of percent of unburned fuel penetrates the motor oil of cars in urban traffic due to incomplete regeneration of the DPF filter [17; 18]. Frequent cold starts, i.e., driving in city traffic or a fault in the injector, will also cause the oil to be diluted with fuel and this may lead to a reduction in the viscosity of the lubricant. Too low oil viscosity means that the lubricating film is too thin and has a low load capacity. In order to increase the safety and reliability of vehicles, the economy of operation, and also for environmental reasons [19; 20], there are currently requirements for continuous oil condition monitoring (OCM), especially for large-volume engines. The OCM [21] is usually part of maintenance plans. Traditional analysis of motor oils consists of a combination of standardized physical and chemical methods that require a variety of laboratory instruments.