To overcome the binary “qualified/unqualified” evaluation inherent in current standards, a quantitative insulation performance evaluation method for mineral insulating oils is proposed in this paper. Ten indicators are selected. Five of them are intrinsic electrical parameters, namely partial discharge inception voltage (PDIV), breakdown voltage (BDV), relative permittivity (εr), dielectric dissipation factor (tanδ), and DC resistivity (ρ). The remaining five indicators are their aging-induced relative variations, denoted as ΔPDIV, ΔBDV, Δεr, Δtanδ, and Δρ, which characterize degradation behavior. Objective weights are obtained from inter-sample standard deviations and incorporated into the Analytic Hierarchy Process. Three widely used oils, labeled A, B, and C, are subjected to 35-day thermal aging at 90 °C and subsequently evaluated. Their comprehensive scores are determined as A (7.934) > B (0.778) > C (0.725), which is consistent with the evolution of oil color during aging. The proposed method overcomes the binary evaluation in current standards, enabling objective and reproducible evaluation of oil performance. It is particularly suitable for evaluating the long-term operational characteristics of oils and provides valuable guidance for material selection in engineering applications.