Castrol Perfecto HT 5 is based on a solvent-refined, high viscosity index mineral oil specially selected for its high thermal and oxidation stability. Perfecto HT 5 is primarily intended for use in closed heat transfer systems with bulk temperatures up to 300°C. It can also be used in open systems at much lower bulk oil temperatures. Perfecto HT 5, because of its solvency and high thermal stability serves as excellent Turbine Flushing oil.