By Ben Visser · July 27, 2022 · 16 Comments

Recently I was talking to an engine rebuilder about oil change intervals for aircraft engines. He related an experience with a plane owner who was having him do an annual inspection on his aircraft.

A problem came up when the mechanic listed an oil change as part of the inspection, even though the aircraft had been flown just 12 hours in the last 12 months.

The owner insisted it did not need an oil change since the manufacturer recommends an oil change only every 50 hours. He completely disregarded the other part of the manufacturer’s recommendation: “Every 50 hours or four months, whichever occurs first.”

There is no good answer for what the oil change interval should be for a super low usage aircraft, like this one that is only flown 12 hours in a year.

I would not expect this owner to change the oil every four hours. But going four years without an oil change will probably result in a ruined camshaft and other problems.

The real problem here is that this is not a rare occurrence in general aviation.

The latest edition of Lycoming Service Letter 270 states that after the initial transition 50 hour oil change, subsequent oil changes should be every 50 hours, but can be extended to 100 hours if unleaded fuel is used. There is an asterisk to a note that adds “or every four months, whichever occurs first.”

So, the 100 hour recommendation would only apply to aircraft flown 300 hours or more a year.

The reasoning behind Lycoming’s recommendation is that when flying on unleaded fuel, the oil does not get lead particles so the owner should be able to go longer between oil changes.

While I know that Rotax engines do have a lead sludging problem, most Lycoming and Continental engines do not.

For example, the University of Illinois Institute of Aviation flight school has run its aircraft with 100 hour oil changes using 100LL for many years. Flight school officials noted they did not have any significant problems, but they did stipulate that the aircraft had to be flown at least 30 hours a month.

In the U.S., the average private plane only flies about 100 hours a year. Many of these pilots ignore the four-month recommendation and use the 50-hour limit for their oil changes.

Lycoming’s service letter should stress the four months oil change interval is more important than the number of hours.

The main lubricant-related failures come from rust and corrosion on camshafts and lifters. This is mainly seen on low usage aircraft that sit idle for extended periods of time.

When airplanes sit for an extended amount of time, surface rust occurs on the cam and the lifters, which is rubbed off and ends up in the oil. Known as rust rouge, it acts like a lapping compound that leads to higher wear and possible failure.

The lead in fuel does not have a significant affect on this process. Therefore, going to an unleaded fuel will not significantly affect the wear rate of the engine. The language in the service letter implies that unleaded fuel will reduce the wear in the engine. The only way this will happen is from poor maintenance leading to oil system plugging.

I understand that aircraft flown over 300 hours a year can go to 100 hours between oil changes when they use unleaded fuel. But they could also do 100 hours between oil changes with 100LL. The secret here is to fly the airplane around 30 hours a month.

Engine manufacturers need to stress the need for high usage of airplanes, not just 100 hours between oil changes.

I have been stressing the importance of changing oil every four months — and not just every 50 hours — for many years.

But many pilots just look at the hours flown as the only criteria for when to change the oil. Then they have problems with their engines.

I know everyone hopes that going to unleaded fuel will cure all of the problems in general aviation. But I do not think it will change that much in the lubricants area.

As long as there are pilots flying only 12 hours a year, there is not going to be full TBO life for all aircraft.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985.

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You want to know how fast rust can form, flew model engines and suspected rust was the culprit. I removed the backplate from the engine right after flying and could see traces of rust forming by the time the backplate was removed. That in turn was ruining my bearings, I would run the engine with a castor oil mix before shutdown solved my bearing problems. Rust forms quick!

sorry about the spelling in in a moving vehicle

ok so please dont listen to bartr. the oil over inactive periods is subject to condensation the naturally occurs due to temperature changes and one result of that is that a type of corrosive is formed in the oil as it sits and can corrode the surfaces. obviously yes when the engine is started that coroded metal or rust wears parts and further contaminates the oil. and yes…oil filters deteriorate over a long period of time and can come apart. but its a long period.

“ The main lubricant-related failures come from rust and corrosion on camshafts and lifters. This is mainly seen on low usage aircraft that sit idle for extended periods of time.” What does this have to do with oil change intervals? The corrosion isn’t caused by old oil it’s caused by inactivity, the engine doesn’t run, oil isn’t circulated onto rotating surfaces, the protective lubricating film flows away and the surface corrodes. Has nothing to do with pouring in new oil. I’m in the camp that says this business about 4 month oil changes is nonsense. The oil doesn’t deteriorate from lack of use and as far as I know there are no objective studies that indicate oil becomes corrosive with time, its a myth like C02 causes global warming.

Let’s review the causes of engine corrosion: a) fuel contains sulfur. Sulfur is a natural component in oil and is mostly removed at the refinery, but some remains. In a humid environment the sulfur hydrates and becomes sulfuric acid. If the oil is changed frequently the sulfur is removed; b) in a humid environment moisture is drawn into your engine crankcase. I’ve seen engine bearings with wiggly lines and/or pitting on them which are caused by corrosion. Water goes to the lowest point in the case and settles in the bearings and rusts them. and at start the rust is scoured away, leaving pits (with sharp edges) and these lines which look like a snail has wiggled across them. c) Oil doesn’t degrade, especially the synthetic component, but the additives do. In order to have a clean engine that is free of water, acid and diminished additives the filter and oil should be changed. I’ve been asked why the filter should be changed if even a little time has been put on the engine since the last oil change? Because the lead that is scoured out by the oil ends up in the filter and if you change the oil without changing the filter then you immediately reintroduce lead into your new oil. Not wise!

And I use Camguard for that reason, even though my aircraft is flown at least once a week for an hour or 2. The engine is now 1,000 hrs past tbo and running great.! [ thanks Mike Bush !]

“its a myth like C02 causes global warming.” I came here to learn something and wasn’t sure which advice was right. Thank you for making it easy for me to tell that I should follow the author’s advice.

Time and hours based on what comprehensive research? My personal experience with hundreds of engines of all types says numbers are mostly based on myths and an ultra conservative approach.

Visual check of the oil at time of preflight is also important.

Oil is cheap, engines are not.

There’s a difference between being economical and being cheap. If anyone won’t change a couple of quarts of oil according to the engine manufacturer’s recommendation (and it is safe to say they do know a “little” about their products), then quite simply, they are being just plain cheap. Guess what – it will come back to bite them or it will bite the next owner of the airplane.

What other “deferred maintenance” is lurking? Hey wow, it costs a lot of money to change the fuel cap gaskets and run that compressor to put air into the tires. If nothing else, when the airplane is sold, the logbook entries will support the claim of “regular maintenance”. Have you ever seen an ad for an airplane which talks about “indifferent maintenance” or “poorly maintained”?

What about a plane that is flown for 1 – 1.5 hours a week and has had Camguard added at the oil change? It won’t be at 50 hours in four months but I would doubt you would see much corrosion in that time with this frequency of usage. Is the 4-month suggestion due to the assumption that lower hours mean that the plane sat for long periods of time? This isn’t necessarily true.

Is there any consideration given to local climate? The aircraft in Pima, Az are in an entirely different climate than those in Galveston, Tx. Unprotected metals in Pima will display less corrosion in one year than those in Galveston do in one week. It’s certainly evident on the airframes.

How did they come up with 4 months, instead of 3 months or 6 months, etc.?

Not only should rust be considered but how about the filter material deteriorating and and causing problems.

Have you ever actually seen an oil filter element that has deteriorated? In 60 years I never have and I’ve pulled apart filters that sat for literally years with oil in them an no use. This is a myth.

Is there a difference in how often the oil should be changed in a turbocharged engine?

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