Here’s an interesting technical write-up on , a now-obsolete but historically significant U.S. military specification. MIL-H-6088: The Hydraulic Fluid That Helped Win the Cold War 1. What Was It? MIL-H-6088 was a U.S. military specification for hydraulic fluid , specifically a mineral oil-based, rust-inhibited, oxidation-inhibited fluid intended for use in ground-based hydraulic systems, particularly those of combat and support vehicles.