The Great General Aviation Oil Debate

By Garth Calitz

Mineral, semi-synthetic, fully synthetic, or the oil your AMO swears by? Few things can turn a perfectly civil gathering of pilots into an engineering dispute quite like asking what goes into the sump.

There are few subjects in general aviation guaranteed to generate as much passionate discussion, unsolicited technical advice and outright certainty as engine oil. Mention the subject at a South African flying club and, within minutes, you will have one pilot explaining why mineral oil is the only sensible choice, another defending his favourite multigrade, an AMO quietly questioning his career choices and somebody in the corner insisting that his engine has run perfectly for 37 years on the same brand. He will then explain that he has never had an engine failure, apparently unaware that this may have more to do with maintenance, operating practices and sheer good fortune than the contents of the oil container.

Welcome to the great general aviation oil debate, where a few litres of lubricant can provoke the sort of tribal loyalty normally reserved for rugby teams, political parties and arguments about whether South Africa's braai etiquette has deteriorated beyond repair.

For an industry that prides itself on engineering discipline, pilots can be surprisingly emotional about what goes into their engines. Oil brands acquire personalities, viscosity grades become matters of principle, and a discussion about lubrication can quickly descend into a competition to establish whose mechanic, overhaul shop or long-departed aviation mentor possesses the ultimate authority.

Fortunately, engines are not particularly interested in clubhouse politics. They have clearances to maintain, temperatures to manage, contaminants to control and moving parts that would prefer not to meet each other without an adequate film of lubricant between them.

Mineral oil: because tradition is apparently a maintenance programme

Straight mineral oil has a legitimate place in aviation, particularly during the break-in period of many new, rebuilt or overhauled piston engines. The objective is to allow the piston rings to seat correctly against the cylinder walls under the conditions specified by the engine manufacturer. In many normally aspirated Lycoming engines, this means starting with non-dispersant mineral oil and changing to an appropriate ashless dispersant oil once oil consumption has stabilised.

This is not an invitation to apply the same rule indiscriminately to every engine. Some turbocharged Lycoming models require ashless dispersant oil from the outset, while Continental engines have their own published requirements. The correct procedure depends on the engine, the work performed and the applicable manufacturer's instructions. The fact that somebody's O-320 was broken in a particular way in 1989 does not automatically make it the universal standard for every piston engine built since.

Yet mineral oil continues to enjoy considerable affection among traditionalists. Something is reassuring about a product that has been used for generations, particularly in an industry where proven procedures matter. The problem begins when respect for experience turns into the belief that anything newer must be a marketing exercise designed to separate aircraft owners from their money.

Sometimes the old method remains entirely appropriate. Sometimes it does not. Engineering has an inconvenient habit of requiring evidence rather than nostalgia.

Ashless dispersant: the grown-up in the room?

For many piston engines that have completed break-in, ashless dispersant, or AD, oil is the everyday workhorse. Its additives help keep contaminants suspended in the oil, reducing the tendency for deposits to accumulate in unwanted places. The oil then carries those contaminants through the lubrication system into the filter until they can be removed during servicing.

This matters in aircraft engines operating on leaded avgas, where combustion products can create deposits that complicate the lives of valves, spark plugs and other components. Aviation lubricants are formulated with the particular demands of these engines in mind, which is why reaching for an automotive oil simply because it carries an impressive specification is not a clever shortcut.

There is also a common misunderstanding worth clearing up. Ashless dispersant describes the additive system, not necessarily the oil's base stock. An AD oil can be mineral-based or, where specifically approved, incorporate synthetic components. This matters because the argument is not simply mineral versus synthetic. It is also about the formulation, specification and suitability of the complete product.

In other words, reading the label is useful. Reading the engine manufacturer's instructions is considerably more useful.

Synthetic oil: the future, or an expensive way to start an argument?

Synthetic oils have genuine technical advantages in many applications, including potential improvements in low-temperature flow and oxidation resistance. Automotive engines have benefited from decades of lubricant development, and it is entirely reasonable to ask whether aircraft engines can benefit from similar advances.

The difficulty is assuming that an oil which performs brilliantly in a modern road car must therefore be suitable for an aircraft engine. Aircraft piston engines operate under different duty cycles, often spend extended periods at high power, and may burn leaded fuel. Their lubrication requirements cannot be reduced to the claim that synthetic is automatically better than mineral.

Nor is the opposite argument, that all synthetic oils are unsuitable for aviation, particularly helpful. Approved aviation lubricants can include synthetic components and certain formulations are permitted for specified engines. What matters is whether the exact product meets the relevant requirements and is accepted for the particular installation.

Lycoming's Service Instruction 1014, Revision P, dated 10 February 2026, sets out its current lubricating-oil recommendations, including applicable classifications and temperature guidance. It also warns that using lubricants not identified as approved by Lycoming is at the operator's risk and may affect engine warranty coverage.

That is a rather more authoritative starting point than a bloke who once worked on a turbine engine explaining why every piston engine should use his preferred automotive lubricant.

W100 or 15W-50? Let the hangar wars begin

Viscosity is another reliable way to turn an otherwise pleasant morning at the airfield into a technical summit. Single-grade oils such as SAE 50 have a long history in aviation, while multigrades such as 15W-50 offer viscosity characteristics intended to accommodate a wider range of temperatures.

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A suitable multigrade may offer advantages during cold starts, when oil flow is especially important. But that does not mean every engine should use one, or that a single-grade oil is somehow obsolete. The appropriate grade depends on the engine manufacturer's recommendations, ambient conditions and the way the aircraft is operated.

South African conditions make the conversation particularly interesting. An aircraft operating in the Highveld may encounter cold early-morning starts, warm afternoons and significant seasonal temperature variations. An aircraft based elsewhere may face a different set of conditions entirely. The correct answer is not to choose an oil based on the temperature displayed on the hangar wall that morning, but to consult the relevant operating documentation.

And please, before anybody declares that the oil used by his flying club is suitable for every aircraft from a Piper Cub to a turbocharged six-cylinder installation, remember that aviation has spent decades establishing engine-specific requirements for a reason.

The forgotten enemy: an engine that never gets properly warm

Perhaps the most important part of the oil debate has surprisingly little to do with brands. It concerns how often the aircraft flies. Some General aviation aircraft are carefully maintained machines that fly regularly; others spend weeks or months in hangars, emerging for a brief engine run that provides the owner with reassurance but may do little to address moisture inside the engine.

Infrequent operation can increase the risk of internal corrosion. Moisture and combustion by-products can contribute to a corrosive environment, particularly when an engine is started but not operated long enough to reach suitable temperatures and drive moisture out of the oil.

Buying a premium lubricant does not solve this problem by itself. Nor does running an engine for a few minutes on the apron necessarily constitute a healthy exercise routine. Owners need to follow the manufacturer's guidance on operation, inspection, oil changes and preservation, taking account of the aircraft's actual usage.

In aviation, the calendar can be just as important as the Hobbs meter. An aircraft with 20 hours since its last inspection is not automatically in better condition than one with 100 hours, and an engine that has barely run is not necessarily an engine that has been protected from deterioration. The most expensive oil in the world cannot turn neglect into preventive maintenance. It can, however, make the invoice more impressive.

There is a reasonable case for consistency, particularly where the engine manufacturer recommends maintaining the same qualified oil after break-in. Continental, for example, has published guidance recommending consistent use of a qualified ashless dispersant oil for its applicable avgas engines.

That does not mean every brand change will produce disaster, or that one manufacturer's product is automatically superior to another compliant formulation. It means that owners should avoid treating each oil change as an opportunity to conduct an informal experiment.

Mixing different oils should not become a routine practice either. If a top-up is necessary, follow the engine manufacturer's instructions and use a suitable product. Record what has been added and maintain a clear service history. An engine logbook should not require a forensic investigation to establish what lubricant has been used over the past year.

The South African reality: price matters, but so does the specification

In South Africa, aircraft owners have to consider more than technical arguments. Product availability, imported lubricant costs, exchange rates and the distance to the nearest supplier can all influence purchasing decisions. A preferred product that is consistently unavailable may be less practical than another correctly specified and approved alternative.

But price alone is a poor selection criterion. The cheapest oil is not necessarily the most economical choice if it does not meet the engine's requirements. Equally, paying a premium does not guarantee a measurable improvement in engine life.

Owners should confirm the applicable specification, check the manufacturer's current guidance and discuss any proposed change with a suitably qualified AMO or engine specialist. They should also ensure that the exact product, not merely the brand name, is appropriate for the engine.

It is a remarkably unglamorous process. Fortunately, engines tend to appreciate unglamorous processes.

So, who wins the oil debate?

The mineral-oil traditionalist has a point during the appropriate break-in procedure. The AD-oil advocate has a point about managing contaminants in an engine in normal service. The synthetic-oil enthusiast may also have a point, provided the formulation is suitable and approved for the application.

What none of them has is a universal answer that applies to every piston engine.

The correct choice depends on the engine, its operating conditions, the maintenance history and the manufacturer's published requirements. Follow those requirements, use the correct oil consistently, monitor consumption and temperatures, and do not mistake an expensive container for a maintenance strategy. The rest is largely noise.

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At the next flying-club gathering, by all means discuss oil. Compare experiences, ask sensible questions and listen to the AMO who actually maintains the aircraft. Just resist the temptation to announce that your preferred lubricant is the only oil worthy of a properly engineered engine. Somewhere nearby, a pilot will be preparing to disagree with you anyway.

The best aviation oil is not necessarily the most expensive, the newest or the one endorsed by the most confident person in the hangar. It is the oil that meets the engine manufacturer's requirements and is used as directed. And if the debate continues long enough, somebody will inevitably ask whether the group should switch to a different brand.

At which point, the AMO will probably recommend changing the subject and the oil when the manual says it is due.