The rheological, ignition, and tribological properties of lubricating oils diluted with biodiesel were analyzed. The flash point t_FP, calculated cetane index CCI, density ρ, coefficient of the temperature density change ε, kinematic viscosity ν, dynamic viscosity η, viscosity index VI, and lubricity during a High-Frequency Reciprocating Rig (HFFR) test (x, y, WSD, and WS_1.4) and lubricating conditions during an HFFR test (oil film resistance FILM and friction coefficient μ) 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% v/v 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 t_FP, ρ, ε, ν, η, and VI, 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 t_FP, ν, and η changes. Simultaneously, the tests showed no significant effect on the lubricity and the CCI. 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 FILM and an increase in μ 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.

This article presents an evaluation of the effect of adding tire pyrolysis oil (TPO) to diesel fuel (DF) on its corrosive properties. Tests were conducted on DF/TPO blends with TPO fractions of 0, 5, 7, 10, 15, 20, and 100% m/m, by determining water content, sulfur content, and acid number, and by conducting a corrosion test on a copper strip. The corrosion test was performed for a standard test duration of three hours and for long-term observations of 10 and 18 days. It was observed that an increase in the TPO content affects the analyzed physicochemical properties of the fuel, which is reflected in changes to its corrosive properties. A relationship is determined between the TPO content in the fuel and the water content, sulfur content, acid number, and the results of the corrosion test on copper strips. The obtained results served as the basis for evaluating the potential use of TPO as a diesel fuel additive, taking into account the requirements regarding the corrosive properties of fuels for marine and automotive applications.