If the Frquel (or Fire resistant Hydraulic Fluid) was used, it must be there for a very good reason. Servo valves are high precision direction equipment and have very very fine clearances . Consequently, they require super clean fluids (NAS 6 – 5). Many of the servo valves manufacturers recommend use Phophate Ester High Resistivity FR fluids, a product I have come across in India is Castrol Anvol PE 46 HR.
Phosphate Ester’s are very sensitive to moisture, and break down to corrosive acids in the presence of moisture/water. In a super heated steam environment the probability of condensate forming is also high. It is imperative, that your arrest the leakage immediately, then deal with other issues. Also re-check composition of seals and gaskets, they must be viton or other synthetic rubbers only.
In conclusion you cannot change FR Fluid in Servo valves with general lube oil.
M Hussam Adeni