An apparatus and method for detecting the viscosity of actuating fluid used to actuate a hydraulically-actuated electronically-controlled injector is disclosed. In an internal combustion engine system including an engine having a crankshaft, a crankshaft angular speed sensor responsive to rotation of the crankshaft, a voltage sensor, and an engine coolant temperature sensor, which respectively generate angular speed, voltage, and temperature indicative signals, are provided. An electronic control module is provided with a memory means, the control module having an associated viscosity indicative signal or parameter whose magnitude is representative of the actuating fluid viscosity and is a function of the angular speed, and voltage indicative signals. The actuating fluid viscosity value is used by electronic control module to vary the actuating fluid pressure and vary a fuel delivery command signal pulsewidth and the timing of the application of the fuel delivery command signal relative to a predetermined crankshaft angular position to control the injection of fuel into engine via the injector independent of engine speed and load.