This paper presents a recent study regarding choosing and optimizing of the appropriate test methods to characterization and analysis of used oils from different sectors of activity, in order to evaluate the heavy metal content. In the first step, the samples preparation stage for metal analysis, a number of preparation methods have been tested and finally chosen as optimal, a microwave digestion method. For comparison the metals were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS). The results obtained by the two methods are comparative and indicate in all analyzed samples the presence of heavy metals such as Fe, Ni, Zn, Cr and Mn. The methods used have been demonstrated to be appropriate by use of the certified reference material for metals in lubricating oils. From the results obtained it is found that the methods of characterization and analysis can be used as optimal and good methods for evaluation of the content of metals in used oils.