ABSTRACT A literature search confirmed that the engineering design for machinery and systems used in cold region environments needs to include an evaluation of the hydraulic oil application. The process begins with a basic understanding of the terms absolute viscosity, kinematic viscosity, cloud point and pour point. The most apparent effect of cold weather on hydraulic oil is the rapid viscosity increase that accompanies the lowering of ambient temperatures. Viscosity Index (VI) defines this effect. Various authors and methods have attempted to use equations, graphs, and tables to compare this response for different fluids. The theoretical work has found practical application in oil classification systems developed by API, SAE and ASTM. Viscosity Index improvers, additives and oil blend formulations can overcome the operating problems associated with low temperature. A discussion of this knowledge is presented with applications to aviation hydraulic oils, motor vehicle oil change intervals, storing marine engines for the winter months, starting cold soaked engines and an empirical equation for refinery formulations of summer, winter and arctic product blends. Guidelines are given to aid in the evaluation and selection of hydraulic fluids for cold region applications. Five major fluid classes are examined. Hydraulic system maintenance and troubleshooting tips complete the presentation.