The use of biodiesels in internal combustion engines leads to tribology concerns because significant oil dilution occurs when biodiesels leak into the oil pan. Dilution by biodiesels may substantially alter oil lubricity and lead to reduced viscosity, which produce tribological changes on engine materials. It is also possible that such oil-biodiesel mixtures undergo further viscosity decreases when subjected to the high temperatures and high contact pressures typical of internal combustion engine boundary lubrication. This research presents new methods to test viscosity reductions because of biodiesel addition in mineral oil and to assess prediction of mixture viscosity considering the chemical breakdown of biodiesel. The results of dynamic viscosity versus dilution rates are presented for small after-tribometer-testing samples of SAE 15W40 mineral oil diluted by four biodiesels (from canola, peanut, and soybean and chicken fat oil). The small sample remaining after tribometer-testing required an ad-hoc method, the suitability of which is discussed; large viscosity decreases in a range of about 70 to 95 % were observed for after tribometer testing of the mixtures. Preliminary data of viscosity versus temperature also are presented for the mineral oil diluted by increasing fractions of different biodiesels and by six pure fatty-acid methyl esters (methyl-oleate, -palmitate, -linoleate, -laurate, -myristate, and -stearate, which are typical components of biodiesels), and they are discussed to explain tribological properties of such mixtures. The results suggest that the viscosity of commercial biodiesels blended in oils could not be fully explained by assuming the former as mixtures of pure methyl esters, which is consistent with the previous experimental data on their tribological effects.