Mineral oil type lubricating oils have an important role in reducing engine wear. But after some period, depending on the
oil quality, oil replacement is needed since the oil loses its properties. In this paper, an engine oil was tested in a 1300
cc engine car and the change of the thermal stability of the collected portions of oil at different usage periods were experimented
by TG/DTG and DSC techniques. Results show that, as usage period increases lubricating oil contains more of the light components
that distill at low temperatures. The increase in the peak maximum temperature, Tmax, of the DTG, as usage period increases, proposes that the molecular mass of the oil increase, which presumably changes the
viscosity characteristics. Moreover as the usage period increases some residue type components, which only can be destroyed
at very high temperatures, are produced in oil phase.