Indiscriminate mixing of MIL-PRF-23699 and MIL-PRF-7808 may result in pressure differences from the published values for either oil.

• A. True
• B. False
• C. Not specified
• D. Rarely

Answer: (A)

Mixing MIL-PRF-23699 with MIL-PRF-7808 changes the oil’s flow and chemical properties in ways the data sheets for each oil don’t account for. Each specification is built around a specific base stock and additive package that defines viscosity at operating temperatures, density, foaming tendency, and compatibility with system materials. When you blend two different oils, the resulting mixture can have a viscosity and flow resistance that aren’t predictable from either oil alone, which alters the oil pump pressure and the pressure seen at bearings and galleries. Additive interactions can also cause emulsions, sludge, or deposits that change flow paths and clearances, further affecting pressure readings. Because of these potential changes, pressures in the system may differ from the published values for either oil. Therefore, indiscriminate mixing can lead to unexpected pressure behavior.