Polyalphaolefin (PAO)-based magnetic fluids are widely used in precision transmission systems for their excellent rheological and lubricating properties, but their stability and magnetic controllability under high-temperature and high-shear conditions remain a key challenge. In this work, a PAO2-based magnetic fluid was prepared via coprecipitation using a sequential modification strategy involving oleic acid and alkenyl succinimide. An energy competition model under multi-field coupling was established using the magnetothermal energy ratio ($\lambda$) and Mason number ($Mn$) to elucidate the system’s rheological behavior. The fluid shows significant shear-thinning behavior under zero magnetic field; a 60 kA/m magnetic field increases the relative viscosity by over 4 times at 5 s⁻¹, while the magnetoviscous effect becomes weak at shear rates over 500 s⁻¹ (corresponding approximately to $Mn$= 1). With increasing temperature, the field-induced viscosity enhancement decreases progressively as thermal disturbance becomes increasingly important. This work reveals the multi-field coupling rheological mechanism, and the results suggest that the OA/T154 modification strategy is a feasible route for obtaining a PAO-based magnetic fluid that remains dispersible and magnetically responsive under the tested conditions. The study provides theoretical and experimental support for the design of intelligent lubricating materials.