Aircraft hydraulic fluids and hydraulic fluid systems have entered a new era. The advent of guided missiles and the fantastic increase in speed and operating altitude of aircraft make present requirements for fluids and systems far more severe than was imagined 15 years ago. This paper presents data on two outstanding fluids developed for use over a wide temperature range. These fluids, Oronite High Temperature Hydraulic Fluids 8200 and 8515, have pushed top operating temperatures upward some 300 degrees F without reducing performance at low temperatures. The article includes commentary by two individuals, Keller and Kleven, along with rejoinder by the authors.