In this work, a complete UV-Vis, IR and (¹H, 13C and DEPT) NMR spectroscopic analysis was performed for a FR3® vegetable oil sample used as dielectric coolant in an experimental distribution transformer. The same spectroscopic analysis was performed for three used FR3® oil samples (i.e., 4 months in use, 8 months in use and 7 years in use), removed from several operating distribution transformers. Comparison of the data indicated that no significant spectroscopic changes, and hence, no structural changes occurred to the oils by the use. Chemical transformations like catalytic hydrogenation (hardening) and hydrolysis were performed to the FR3® oil sample and the obtained products were analyzed by spectroscopic methods in order to collect further structural information about the FR3® oil. Accelerated aging tests in laboratory were also performed for three FR3® oil samples affording interesting information about the structure of the degradation products. These findings would be valuable to search for a spectroscopy-based technique for monitoring the life-time and performance of this insulating vegetable oil.