The large volumes of waste oils from electric power generation systems has intensified their disposal concerns, and as alternatives, regeneration and reclamation of the oils are environmentally preferred. This study investigates the regeneration of contaminated transformer oil using mesoporous γ-alumina adsorbents synthesized from cost-effective boehmite. Two synthesis routes, thermal–chemical (TC) and chemical–thermal (CT), were evaluated using Taguchi’s L16 orthogonal array to optimize the preparation conditions. The performance of the synthesized adsorbents in oil regeneration was then assessed through a one-step batch adsorption process and multiple saturation cycles. Textural characterization by BET analysis identified the optimal TC-derived γ-alumina with a specific surface area of 226.76 m² g⁻¹, a pore volume of 0.25 cm³ g⁻¹, and an average pore diameter of 5.95 nm. The optimized adsorbent demonstrated superior physicochemical performance in treated oil, significantly improving the total acid number (TAN), dielectric dissipation factor (DDF), and electrical resistivity. In addition, it exhibited low mechanical erosion (3.35% within a particle size range of 0.710–0.850 mm), outperforming comparable commercial products (~7.04% erosion). FTIR and GC–MS analyses confirmed the effective removal of key contaminants, including 1,2-benzenedicarboxylic acid derivatives (diisooctyl ester and mono-(2-ethylhexyl) ester). Based on the latest literature, the comparison of thermal–chemical and chemical–thermal synthesis routes, along with their effects on pore-size distribution and the attrition strength of γ-Al₂O₃ adsorbents, represents a novel aspect of this study and warrants further investigation to maximize adsorption performance.