When should I replace the heat medium oil for thermal fluid heater system?

The life of the heat medium oil for thermal fluid heater is related to the operating temperature, and the life is shorter when the operating temperature is high.
After half a year, the heat medium oil is used for sampling every 2-3 months. When the flash point of the heat medium oil changes by more than 20%, the acid value is less than 0.5 mgKOH/g, and the residual carbon is less than 1.5 wo/o, when the color of oil becomes dark, it indicates that it has failed and needed to be replaced.
The relationship between heat transfer oil life and temperature is shown in the following table: (unit, year)