A system includes a housing. The system includes a filter element disposed in the housing. The filter element is configured to filter an operating fluid. The system includes an additive disposed in the housing. The additive is configured as a crystalline solid at a temperature below 60 C. and configured to disperse into an operating fluid at a temperature in a range of 60 C. to 160 C. The additive can improve the oxidative stability of the operating fluid.