Not sure what the end result you are looking for. The hydraulic fluid used in the Twotter is the standard general aviation red stuff MIL-H-5606. The Shell Oil web page (sorry, too compuker-illiterate to set up a hyperlink)says their version (Shell 4) has a pour point of below -75 F. The Canadian Operators Kenn Borek Air took a pair of Twotters down to the Antarctic mid-April this year, and had no reported problems in -68 C.