In many applications, mineral oil based heat transfer fluids can perform as well as synthetic fluids.
The fact that the ring structure of the original di-phenyl/ di-phenyl oxide chemistry and its alkylated aromatic cousins (employed in synthetic fluids) is less prone to thermal degradation than the aliphatic structure of mineral oils has been well established. However, replacement rate and its in-verse, life expectancy, are affected by more than a fluid's thermal stability. While any number of operating problems can cause fluid to experience abnormally high degradation rates, it is the mechanical and the hardware issues that typically control how long a fluid remains in a system. For example, any action that results in opening a line -- for example, a pump or valve replacement -- seldom is accomplished without fluid loss. In many cases, these maintenance issues occur with increasing frequency as the system ages. On average, users replace 10% of the fluid charge per year as a result of ongoing maintenance issues. Expanding or reconfiguring a system also typically results in substantial fluid replacement.