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Which Oil Should You Use for Your Aircraft Piston Engine?

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Lubrication Is Not the Only Function of Your Aircraft Engine Oil.

Aircraft engine oil plays a crucial role in engine performance and longevity. While lubrication is important, it is not the oil’s only function. In fact, it may not even be the most critical function in some cases. Choosing the right oil for your aircraft piston engine ensures optimal performance, reduced wear, and improved reliability.

Functions of Aircraft Engine Oil

Aircraft engines operate in demanding conditions, and engine oil must perform multiple functions beyond lubrication:

Lubrication to Reduce Friction and Wear

Oil reduces friction between moving parts, preventing excessive wear. Depending on engine location, it provides different types of lubrication:

  • Hydrodynamic Lubrication – When moving parts are separated by a film of oil, like hydroplaning on water. This happens under low load and high speed.
  • Boundary Lubrication – Under high loads and low speeds, oil forms a thin layer that prevents metal-on-metal contact. Anti-wear additives help in these conditions.
  • Mixed Lubrication – A combination of both types.

Cleaning

Aircraft piston engines accumulate contaminants from combustion. Large tolerances, big displacement parts, and low RPMs in air-cooled engines lead to high blow-by. This means more contaminants enter the oil.

Using avgas with leaded fuel further complicates the issue. Lead sludge formation is a major problem, as seen with Mobil AV 1, the first synthetic oil in aviation, which led to many ruined engines due to lead sludge deposits.

Corrosion Protection

Oil creates a protective layer that prevents moisture and acids from causing internal engine corrosion. This is especially important for aircraft that sit idle for long periods.

Cooling

Oil helps dissipate heat, cooling internal parts like pistons, which experience extreme temperatures during combustion.

Sealing

Oil helps seal piston rings, reducing blow-by and maintaining compression for better performance and efficiency.

Actuation

Oil plays a role in operating key engine components such as:

  • Propeller Governors – Controls the constant-speed propeller system.
  • Turbocharger Wastegates – Regulates turbo pressure to optimize engine power output.

Mineral, Semi-Synthetic, or Full Synthetic Oil?

Aircraft engine oils come in three main types:

  • Mineral Oil – Used for break-in and preservation. It provides good lubrication but lacks advanced additives.
  • Semi-Synthetic Oil – Offers better performance but struggles with lead deposits in high-displacement engines with small oil sumps.
  • Full Synthetic Oil – Could become viable with unleaded fuel, but current synthetic oils fail to handle lead deposits effectively.

Single-Grade vs. Multi-Grade Oils – Understanding Viscosity Ratings

Viscosity describes an oil’s resistance to flow. Aircraft engine oils are either single-grade or multi-grade:

  • Single-Grade Oils – Maintain a stable viscosity at high temperatures, making them ideal for warm climates and stable operating conditions.
  • Multi-Grade Oils – Contain viscosity index (VI) improvers, allowing them to flow better at cold temperatures while maintaining viscosity at high temperatures.

For example, SAE 20W-50 oil behaves like an SAE 20 at 0°C (32°F) but maintains viscosity equivalent to an SAE 50 at 100°C (212°F). Multi-grade oils are a trade-off because viscosity improvers do not have great lubrication properties.

Additives in Aircraft Engine Oil

Aircraft engine oils contain specialized additives to enhance performance:

  • Viscosity Improvers (VI) – Stabilize oil flow at different temperatures.
  • Anti-Wear Additives – Protect against metal-on-metal contact under high load.
  • Friction Modifiers – Reduce internal friction, improving efficiency.
  • Corrosion Inhibitors – Prevent rust and corrosion during inactivity.
  • Tackiness Agents – Help oil cling to metal surfaces.
  • Acid Neutralizers – Reduce acidity from combustion by-products.
  • Oxidation Inhibitors – Prevent oil breakdown over time.
  • Dispersants – Keep contaminants suspended in the oil to prevent sludge formation.

Aviation oils do not contain traditional detergents, unlike automotive oils. Instead, they use ashless dispersants to maintain cleanliness without forming harmful ash deposits.

Straight Mineral vs. Ashless Dispersant (AD) Oils

  • Straight Mineral Oil – Used for engine break-in and preservation, lacks dispersants.
  • Ashless Dispersant (AD) Oil – Used for normal operation, helps keep engines clean.

Oil and Oil Filter Change Intervals

Regular oil changes prevent sludge build-up and extend engine life. Recommendations vary by engine type:

  • Continental and Lycoming: Every 50 hours (or 25 hours without an oil filter) and at least every four calendar months.
  • Rotax: Every 100 hours with unleaded fuel or 50 hours with leaded fuel.

Choosing the Right Oil Based on Application – Different Operating Conditions Require Specific Types of Aircraft Engine Oil

Normal Operation (Continental and Lycoming)

Single-Grade Oils (Warm Climates & Stable Temperatures):

  • AeroShell W100 (SAE 50)
  • AeroShell W80 (SAE 40, for cooler temperatures)
  • Phillips 66 Type A 100 (SAE 50)
  • Phillips 66 Type A 80 (SAE 40)

Multi-Grade Oils (Variable Climates & Cold Starts):

  • Phillips 66 X/C 20W-50 (fully mineral-based, highly recommended)
  • AeroShell W15W-50 (semi-synthetic, but not for break-in)
  • Total Aero DM 15W-50

Break-In Oils (New or Overhauled Engines)

  • AeroShell 100 (Mineral) – SAE 50
  • AeroShell W100 (Mineral, AD) – SAE 50
  • Phillips 66 X/C Mineral 20W-50 – Alternative break-in oil

Engine Preservation (As per Tanis Aircraft Products Preserving Kits, requiring MIL-L-6082C / SAE J1966, Grade 1100.)

  • AeroShell Oil 100 (Mineral) – SAE 50 (meets MIL-L-6082C / SAE J1966, Grade 1100)

Rotax Engines

  • AeroShell Sport Plus 4 – Standard Rotax oil
  • XPS 5W-50 Full Synthetic – Required for Rotax 915/916 engines

Final Recommended Aircraft Engine Oils

Single-Grade Oils (SAE J1899 – Ashless Dispersant)

  • ✅ AeroShell W100 (SAE 50) – Standard AD oil for warm weather.
  • ✅ AeroShell W80 (SAE 40) – Lower viscosity for cooler climates.

Multi-Grade Oils (SAE J1899 – Ashless Dispersant)

  • ✅ Phillips 66 X/C 20W-50 – Fully mineral-based, highly recommended if single/grade oil cannot be used.

Break-In Oils (SAE J1966 – Mineral Oil, Non-Dispersant)

  • ✅ AeroShell 100 (Mineral) – SAE 50 (break-in oil, no additives).
  • ✅ AeroShell W100 (Mineral) – SAE 50 (includes ash dispersant).
  • ✅ Phillips 66 X/C Mineral 20W-50 (alternative break-in oil)

Recommended Oils for Engine Preservation

  • ✅ AeroShell Oil 100 (Mineral) – SAE 50

Recommended for Rotax Engines

  • ✅ XPS 5W-50 Full Synthetic

Use of CamGuard

  • ✅ Recommended for normal operation to enhance anti-wear protection, particularly for Lycoming camshaft longevity and corrosion prevention.
  • 🚫 NOT recommended during engine break-in or for preservation since straight mineral oils should be used in these cases.
  • 🚫 NEVER use friction modifiers for Rotax engines, as they can cause the overload clutch to slip.

Conclusion

Choosing the right aircraft engine oil depends on your operating environment, engine type, and application. While lubrication is critical, oil also plays key roles in cleaning, cooling, and corrosion protection. Regular oil changes and the right additives can significantly extend the life of your aircraft engine.

About Quest Aeronautics

Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.

About Author

Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.