What is the unacceptable fuel percentage in L/O?

• A. 5.0% and greater
• B. 3.5-4.9%
• C. 2.0-3.4%
• D. 0-1.9%

Answer: (A)

Fuel dilution in lube oil is what this is assessing. When fuel finds its way into the oil, it lowers the oil’s viscosity and its film strength, which reduces lubrication and heat‑carrying capacity. As the fuel percentage climbs, the oil becomes too thin to properly protect bearings and components, increasing wear and the risk of engine damage. The threshold at which this condition is considered unacceptable is 5.0% by volume or greater; at or above this level, corrective action is required, such as diagnosing fuel system leaks or faults, changing the oil, and ensuring the fuel–air mixture and fuel control are functioning correctly. Lower ranges (below 5%) are typically within acceptable limits per standard maintenance specs, but ongoing oil analysis is used to monitor trends.