In hydraulic active suspension systems, an often overlooked source of nonlinearity is the performance deviation of the working hydraulic oil through the wide band of operating temperatures. In this paper, a quarter-car, Simulink® Simscape nonlinear hydraulic active suspension model has been constructed, taking ISO VG 22 oil as the working oil[29]. A PID controller has been tuned for the active suspension system. Targeted response parameters were tire deflection and suspension travel, as measures of vehicle handling, and sprung-mass acceleration as a measure of riding comfort. At a typical moderate vehicle speed of 40 km/hr, system response was investigated for both a single sinusoidal bump and a continuous wavy sinusoidal road inputs. Simulation was performed at two distant, extreme, operating temperatures, namely -30°C and 60°C. Results showed that at higher temperatures, the decrease in oil viscosity of the hydraulic actuator results in higher tire-rebound amplitudes and longer settling times, and a higher sprung mass acceleration amplitude, worsening both tire-road holding and ride comfort compared to those at -30°C. The most significant conclusion is that an active suspension's hydraulic actuator actually behaves as a nonlinear damper, besides being a force generator.