Two other methods of validation were also employed: the use of reference material and acid digestion procedure. The certified reference material wear-metals in lubricating oil—NIST SRM 1084a—that has a certified value only for magnesium, was analyzed by the proposed method and the magnesium concentration found (94.9±2.6gg−1) did not differ significantly from the certified value (99.5±1.7gg−1) at a 95% confidence according to Student’s t-test. Because no other reference material was available in our laboratory, in order to validate the results of the other elements we also investigated a digestion procedure, based on a procedure described by Amorim et al. [31]. In this work, the digestion was performed under reflux using a “cold finger” to avoid excessive evaporation of the acids and reduce the risk of contamination. The total time of digestion was approximately 6 h. The other details of the acid digestion were given in Section 2.5. The results of the three selected samples subjected to the acid digestion are presented in Table 3. These values did not differ significantly from the values obtained by three-component solution and dilution methods, according to the Analysis of Variance (ANOVA) at the 95% confidence interval. This shows that the proposed method provided accurate results, although it is very simple and fast. Another important point is that concentrations of Ca, Mg and Zn found in new lubricant oils are usually much greater than the limits obtained in this work. Thus, very low detection limits and high sensitivity techniques are not required for this application, as confirmed before [32]. The overall result was very satisfactory for routine analysis.