Definition
The deliberate thinning of engine oil by introducing a measured amount of fuel (gasoline) into the oil system before shutdown, so the oil flows more easily during the next cold-weather start. The fuel evaporates out of the oil once the engine warms up after starting, returning the oil to its normal viscosity.
Plain English
Mixing a little fuel into the oil before shutting down in cold weather, so the oil is thin enough to flow when you start the engine again. Once the engine warms up, the fuel boils off and the oil goes back to normal.
Context Anchor
Seen in aircraft maintenance manuals, cold-weather operating procedures, and discussions of piston engine starting in low temperatures.
Derivation
Dilution comes from the Latin diluere, meaning to wash away or thin out. Here it describes the oil being thinned — not contaminated — on purpose.
Why Pilots Care
Ensures adequate oil flow on initial startup in cold conditions, reducing wear on engine components until normal operating temperature is reached.
Grounding Statement
Cold oil can become so thick that it moves slowly, and oil dilution helps it flow during the next start.
Intuition Check
Oil dilution does not mean accidental contamination of the oil. In this context, it means an intentional, approved method of thinning the oil for cold-weather starting.
Example Sentence 1
Before shutting down on a cold winter night, the mechanic ran the oil dilution system to make the next morning's start easier on the engine.
Example Sentence 2
After engine start the pilot monitored oil temperature until dilution was no longer required and normal viscosity returned.