FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationEasy
A mechanic reads an oil container labeled 'SAE 15W-50.' What does this designation indicate about the oil?
- AIt is a multi-viscosity oil that behaves like a 15-weight oil when cold and a 50-weight oil when hot
- BIt must be used only in engines rated for 50 hours between oil changes
- CIt contains 15% synthetic and 50% mineral base stock
- DIt is a single-grade oil that only meets viscosity requirements at 50°F
Show answer & explanationAnswer & explanation
Correct answer: A. It is a multi-viscosity oil that behaves like a 15-weight oil when cold and a 50-weight oil when hot
The 'W' indicates the oil's cold-temperature (winter) viscosity rating, and the second number indicates the viscosity at normal operating temperature. A 15W-50 oil flows like a thinner 15-weight oil at cold start and provides the protection of a 50-weight oil once at operating temperature.
Why the other options are wrong
- B. The designation has nothing to do with oil change intervals.
- C. The numbers refer to viscosity ratings, not percentage composition of base stocks.
- D. This describes single-grade oil incorrectly; the W and dual numbers indicate a multi-viscosity oil.
Multi-Viscosity Oil Grades
Oils like 15W-50 have additives allowing them to maintain proper viscosity across a wide temperature range, flowing easily when cold and maintaining film strength when hot.
- W = winter/cold viscosity rating
- Second number = viscosity at operating temperature
- Multi-grade oils reduce cold-start wear
- Single-grade aviation oils are labeled simply, e.g., SAE 50
Memory trick: 15W-50: Winter-thin, Summer-thick, same bottle