In general this statement is technically sound, but it needs some clarification. For mineral turbine and transformer oils, 60 °C is a conservative long-term operating limit, not an absolute physical limit. Short-term heating to 65-70 °C during purification or drying is usually acceptable if oxygen exposure is controlled. The real driver of degradation is the combination of temperature, oxygen, and time. Above about 60 °C, oxidation rate increases rapidly, so for continuous operation it is good practice to stay below this level to preserve antioxidant reserve and slow acid and sludge formation.