The application of used engine lubricating oils for further service was evaluated on the basis of chemical and thermal analyses. The most substantial chemical variables, namely kinematic viscosity at 323 and 373 K, flash point and content of foreign solids and oxide ash were determined. The two groups of thermal variables were determined based on the TG and DTG curves. The temperatures for onset, end and successive mass losses were read from the TG curves, whereas the temperature range of DTG peak, peak temperature, peak height and peak width at a half of peak height were read from the DTG curves. To find the relation between the chemical and thermal variables, regression and principal component (PCA) analyses were applied. Good linear relations were found between majority of chemical and thermal variables. The results of PCA indicate that the TG and DTG techniques are very useful in defining the degree of wear of used lubricating oils.