Sludge-resistant heat transfer fluid has substantial oxidation resistance and extended service life. This zero-pressure fluid provides precise, uniform temperature control to 550°F in closed-loop thermal oil systems where the heat transfer fluid is more than occasionally exposed to air. The crystal clear OR fluid is comprised of a unique high-stability base plus high-performance oxidation inhibitor/stabilizer. Performance of the OR fluid is particularly impressive under the same grueling conditions that quickly cause severe oxidation, and sludge formation in conventional heat transfer fluids.