A brief review of the methods for determining the total base number of engine oils (TBN) is presented. This integral indicator characterizes the total content of basic impurities (additives) in terms of the amount of potassium hydroxides in one gram of engine oil (mg KOH/g). Used engine oils are usually titrated with hydrochloric acid in a non-aqueous medium, the potential of the indicator electrode being under control. The same technique can be successfully used in analysis of commercial (fresh) oils. Titration curves are recorded with an indicator glass electrode. TBN of fresh and used engine oils are determined with approximately the same convergence (Sr < 2 %). To check the compatibility of redetermined TBN values, the repeatability and reproducibility limits specified in GOST 11362–96 were used. Using the M-10G2 oil as an example, different variants of potentiometric titration are compared, the corresponding metrological characteristics were determined, and recommendations for testing laboratories were developed. Titration of oil to a certain fixed potential recommended in GOST 11362–96 results in underestimated TBN values compared to other options used for identification of the equivalence point and worsens the reproducibility of the results when using samples of different weights. We recommend to determine TBN values for fresh or used engine oils by direct potentiometric titration using three-tangent method. The accuracy of the obtained results is checked by the standard method of additives (spike recovery test); the error of determination being less than 15 % rel. It is shown using Shewhart maps, that the metrological characteristics of the repeatability and reproducibility are stable, but the results of analysis of control samples often go beyond the warning limits, especially for the samples of used oil. Promising directions of future studies are proposed.