Aviation piston engine oil, is a specialized aircraft lubricant designed to protect and enhance the performance of piston engines in aircraft. Unlike automotive engine oils, aviation piston oils are formulated to withstand extreme temperature variations, high-altitude conditions, and the unique combustion characteristics of aviation gasoline (avgas). These oils ensure optimal engine performance, longevity, and safety by minimizing wear, reducing heat, and preventing the buildup of harmful deposits.
Below are some hydraulic fluids that can used in aviation:
Piston engine aircraft primarily use two types of oils:
Aviation piston oils come in single-grade and multi-grade varieties. Multi-grade oils, such as 15W-50, provide better cold-start lubrication and stability in varying temperatures, while single-grade oils like SAE 50 are more suited for steady operating conditions.
Single-grade aviation oils have a fixed viscosity at a given temperature, making them more stable in consistent weather conditions but less adaptable to temperature fluctuations. Multi-grade oils, on the other hand, contain additives that allow them to flow better in cold temperatures while maintaining viscosity at higher temperatures. This makes multi-grade oils more versatile, especially in varying climate conditions.
The selection of engine oil depends on the type of aircraft, operating environment, and manufacturer recommendations. Common aviation piston engine oils include:
These oils are specifically designed to handle the high stress and operating temperatures of aircraft engines, ensuring smooth operation and extended engine life.
A piston engine oil’s function inside a piston engine is to:
AeroShell Oils and AeroShell W Oils are intended for use in four-stroke (four-cycle) aircraft reciprocating piston engines. They are not recommended for use in automotive engines converted for use in aircraft, and in these cases the conversion shop should be consulted for proper oil recommendations. The term “ashless dispersant” was given to aviation oils to distinguish them from straight mineral aircraft piston engine oils. Automotive and heavy duty truck engine oils contain ashless dispersants and ash-containing detergents. They were 7273 traditionally called detergent oils (some aircraft operators incorrectly refer to ashless dispersant oils as “detergent oils”).
Ashless Dispersant (AD) oil is a type of aviation oil formulated to keep contaminants suspended in the oil, preventing sludge and deposit formation. Unlike conventional oils, AD oils do not contain metallic additives that could leave ash residue, reducing the risk of engine deposits and wear. This makes them ideal for piston-engine aircraft, improving cleanliness and overall engine performance.
Viscosity determines the oil’s resistance to flow and its ability to lubricate engine components under various temperatures. Proper viscosity ensures smooth engine operation, reducing friction and wear while maintaining oil film strength. Low-viscosity oils provide better cold-start performance, while higher-viscosity oils offer superior protection at high operating temperatures
Aircraft engine oil performs several key functions:
Mixing different brands or types of aviation oils is not recommended unless specified by the manufacturer. While some oils may be compatible, others may contain different additive packages that could negatively impact engine performance, lubrication properties, and longevity.
For the majority of aircraft piston engines the selection of the right grade is important to maximise engine performance and engine life.
Running-in use AeroShell Oils
Normal operation use AeroShell W or W Plus Oils
Mineral multigrades use a lightweight mineral oil (the same as a lightweight monograde oil), but include an additive called a Viscosity Index Improver. The best way to visualise this viscosity index improver is as a long chain molecule which curl up like a ball of string when cold, but then uncurls as the temperature increases.
2. Semi-Synthetic Multigrades
When Shell first started evaluating multigrade aviation piston-engine oils over 30 years ago, testing proved that multigrade oils formulated only with mineral base oils did not have adequate base oil viscosity (thickness) to properly lubricate all the high load points in the engine. Then we tested and flight evaluated a formulation made with all-synthetic base oils. This formulation had excellent anti-wear characteristics in all tests run. However, in the flight evaluations, some engines would reach 600 t900 hours then lose oil consumption control and/or compression. When the engines were disassembled, we found that the piston rings were covered with a grey tacky substance that was primarily made up of the lead by-products of combustion (from the use of leaded aviation gasoline). Although synthetic oils are excellent lubricants with good high-temperature stability and very good low-temperature flow characteristics, they are relatively poor solvents. In an aircraft engine, the lead by products of combustion must be dissolved by the base oil so they can be carried away from the ring belt area and removed from the engine when the oil is changed.
Semi-Synthetic oils use a blend of mineral oil and synthetic hydrocarbon oil. Due to the naturally high viscosity index of synthetic oil – its viscosity changes less with temperature as compared to mineral oils – there is no need to add a viscosity index improver.
Piston engines operate on the four-stroke cycle, which includes:
Lubrication plays a crucial role in reducing friction, preventing overheating, and ensuring smooth piston movement.
Piston engines in aviation can be categorized into:
Each type has unique lubrication needs, making the selection of the right piston oil essential for efficient engine operation.
Most engines have nitrided steel cylinder liners and chrome-faced piston rings. Whilst this combination will often break-in quite easily, it would be good advice to fly as often as you can in the initial break-in period if your engine is fitted with steel linersrather than Channel Chrome bores.
The steel liners are particularly susceptible to surface corrosion in the early life of the engine, surface rust being quite common after only a few days of inactivity if the conditions are right. The straight oil is used for 50 hours in these engines more to create a thin protective film of lacquer on the bore rather than to hasten break-in; the break-in process tends to happen quite quickly with steel bores, but the potential for corrosion remains.
The Channel Chrome cylinders obviously do not suffer with the same corrosionproblem, but the hard chrome surface is much more difficult to break-in – sometimes taking over 100 hours to break-in. Therefore it is very important to be patient to ensure proper ring matching with corrosion-resistant cylinders, as the hard surface of chrome bore engines is much more prone to cylinder glazing following improper break-in.
Radial engines utilise special parts and, depending upon the type of aircraft, application and climate are often subject to specific problems not seen in other types of piston engines.
In a radial engine each bank of cylinders has all of the cylinders in the same plane and transmits power through a single master rod bearing to the crankshaft. This master rod bearing is subjected to high loading and absorbs the shock and vibration from the cylinders and thus requires very good protection from the lubricant. Generally radial engines have greater piston and bearing clearances and thus require a higher viscosity oil.
Vintage aircraft piston engines, including vintage radial engines, were approved on oils produced at the time the engine was originally manufactured. Many of these oils are no longer available. If the engine was approved on an aviation oil other than a MIL-L-6082 or a MIL-L-22851 oil then operators should consult with either the engine rebuilder or oil supplier. On no account assume that present oils are direct replacements for old vintage aircraft applications.
There are several clues that the engine will give you, and one key one is oil consumption, so you should really start to take note what the consumption is from the start. What you will find is that the consumption will probably be quite high initially, will reduce rapidly and then plateau at a certain value. What this value is, is not really too important – it can be anywhere in the range of 1 litre every 4 to 20 hours – an indication of stabilisation is more the key. Too high an oil consumption indicates that the engine has not broken in yet (or has possibly glazed if it is over 100 hours operation).
Second indication to look out for is the exhaust stack. This will normally start being black and wet (due to the high level of oil burned during the initial stages of break-in). It will then turn to black soot and finally produce a tan / grey deposit, indicating that there is little oil being burned and the mixture setting is correct.
Another indication is that of crankcase pressurisation. If you fill the engine up to the maximum oil level indication and it rapidly loses the first half litre down the breather pipe, then many people just fill the engine with less oil next time. This is fine if it is an old, worn engine, but during break-in it is actually telling you something.
Assuming that it is not an aerobatic engine, the reason that the oil is being pushed down the breather is that the crankcase is being over-pressurised by exhaust gas getting past the ring pack. In other words, the engine is not effectively sealing itself and has not achieved a good gas seal between rings and bore – so the break-i process is not yet complete.
It is best to top oil up to the maximum and monitor whether it rapidly loses the firs half litre or so.
Regular oil analysis can help detect issues early and determine the optimal change interval.
