I have heard of this problem with a Westinghouse system and there the problem was that the turbine oil was used as make-up in the Steam Generator PORV reservoirs. Determine what procedures are in place to prevent mistakes. Dedicated containers can help as would signage on the equipment to use only phosphate esters. Too much mineral oil affect the fire resistance, can affect EPR seals and can affect fluid life.
A possible contributing factor is that Fyrquel fluid is normally only in drums and typically the only drum for make-up is down near the ST control system reservoir and might require an Operator to operate the drum pump or fill cart. Having phosphate ester fluid in suitable pails or other containers might help. Do not use PVC, or galvanized containers.