The hydraulic system may use either a phosphate-ester-base synthetic fluid such as Skydrol, or mineral-base MIL-H-5606 fluid. Which two factors must be noted and followed?

• A. Viscosity and pressure
• B. Color and age
• C. Quantity of fluid and type
• D. Temperature and limit

Answer: (C)

When a hydraulic system can use either a phosphate-ester synthetic fluid like Skydrol or a mineral-base MIL-H-5606 fluid, the two essential factors to note are the exact fluid type and the amount of fluid in the system. The type determines compatibility with seals, elastomers, metals, paints, and overall performance (including fire resistance and temperature limits). Using the wrong fluid can cause seals to swell or degrade, components to corrode, or incorrect operation due to different viscosity and additive packages. The quantity matters because it governs proper system servicing, the amount needed to fill, bleed, and maintain correct pressure and operation, and to avoid issues from under- or overfilling. If switching types, a thorough drain and flush are often required to prevent contamination and ensure the system runs with the chosen fluid. While other factors like viscosity, temperature, or color can be relevant in other contexts, the immediate requirements for systems designed to accept either fluid are knowing the exact type and maintaining the correct quantity.