## 4 Discussion and Conclusions  
Photothermal techniques provide enough precision to measure thermal properties with errors lower than 2%; this precision makes it possible to obtain the variation of thermal properties when vegetable oils are added to motor oil.  
Vegetable oils as additives in motor oil can modify thermal diffusivity values for the obtained mixtures; this can be useful depending on the application. During this research, different vegetable oils percentages in the range of 5% to 20% were used; the carboxyl groups in the bio lubricant offered improved lubricity compared to commercial grade SAE15W40 lubricant and potential next-generation lubricants.  
It was concluded that the thermal properties of machinery oil can be changed by adding an appropriate percentage of vegetable oil; sunflower and almond oils can raise the thermal diffusivity, while rapeseed oil lowers the thermal diffusivity. Furthermore, the viscosity coefficient is closely related with the thermal properties, because the modification of thermal properties implies a modification of lubricant properties. Photothermal techniques provide an alternative method for indirectly determining the lubricant properties of oils. This alternative method has the advantage of avoiding the use of mechanical parts, which are susceptible to failing.  
The influence of vegetable oils on the thermal properties of lubricating oil is closely related to the number of fatty acids. Rapeseeed oil raises the thermal conductivity in the same manner as jatropha oil; in both cases, the increase in thermal diffusivity can help in cooling applications. On the other hand, almond and sunflower oils lead to a reduction of thermal diffusivity, like castor oil. The content of vegetable oils is a crucial factor in tunning the thermal parameters of lubricant oils, which are generally used as gear oils, automatic transmission fluids, power steering fluids, and hydraulic fluids.  
Future research into tribology is proposed to establish a possible wear scar reduction. Other options could be to explore viscosity and thermal properties as a function of temperature in future investigations.  
Conceptualization, C.J.T.R. and G.L.H.; Data curation, O.O.S.G. and J.P.R.J.; Formal analysis, J.d.J.A.F.C. and G.L.H.; Methodology, J.d.J.A.F.C. and C.O.G.M.; Resources, J.P.R.J.; Software, O.O.S.G. and C.J.T.R.; Validation, C.O.G.M.; Writing--original draft, G.L.H.; Writing--review and editing, J.d.J.A.F.C. and G.L.H. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data that support the findings of this study are available on request from the corresponding author.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**Fatty Acids (\%)**} & \multicolumn{2}{c}{**Density\(\times\)10\({}^{2}\)**} & \multirow{2}{*}{**Reference**} \\  & **Oleic** & **Linoleic** & **(gr\(\cdot\)cm\({}^{-3}\))** & \\ \hline Sunflower & 15 & 62 & 9.16 & [34] \\ Almond & 34 & 46 & 9.21 & [35] \\ Rapeseeed & 61 & 21 & 9.14 & [34] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Fatty acids content and density for the studied samples.