| Summary: | The rheological, ignition, and tribological properties of lubricating oils diluted with biodiesel were analyzed. The flash point <i>t<sub>FP</sub></i>, calculated cetane index <i>CCI</i>, density <i>ρ</i>, coefficient of the temperature density change <i>ε</i>, kinematic viscosity <i>ν</i>, dynamic viscosity <i>η</i>, viscosity index <i>VI</i>, and lubricity during a High-Frequency Reciprocating Rig (HFFR) test (<i>x</i>, <i>y</i>, <i>WSD</i>, and <i>WS</i><sub>1.4</sub>) and lubricating conditions during an HFFR test (oil film resistance <i>FILM</i> and friction coefficient <i>μ</i>) were determined. The test was performed for the oil mixtures of the lubricating oil of the SAE 30 and SAE 40 viscosity grades, which were diluted with the biodiesel blend (D93B7—diesel oil with 7% <i>v</i>/<i>v</i> fatty acid methyl esters, FAME) at concentrations of diesel oil in the mixture equal to 0% (pure lubricating oil), 1%, 2%, 5%, 10%, 20%, 30%, 50%, and 75% m/m, respectively. The experiment confirmed the existence of clear relationships between the increase in the dilution of lubricating oil with tested biodiesel blend and <i>t<sub>FP</sub></i>, <i>ρ</i>, <i>ε</i>, <i>ν</i>, <i>η</i>, and <i>VI</i>, and the deterioration of lubrication conditions. It is recommended to take remedial action even in the case of low diesel oil concentration (<5% m/m) in the lubricating oil due to <i>t<sub>FP</sub></i>, <i>ν</i>, and <i>η</i> changes. Simultaneously, the tests showed no significant effect on the lubricity and the <i>CCI</i>. The critical contamination of oil with fuel in the range of 2–5% by weight, as indicated in the literature, still allowed for a certain “safety margin” regarding these parameters. However, when the concentration of diesel fuel in the lubricating oil exceeded 5–8% m/m, the deterioration of the lubrication was expressed by a decrease in <i>FILM</i> and an increase in <i>μ</i> was observed; hence, such a contamination should be considered excessive. When the concentration of diesel fuel exceeds 10% by weight, there is a serious risk of engine damage during operation.
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