Some Practical Advice – Which Engine Oils to Use

PracticalImages: Courtesy & Copyright Dave Marion

Some Practical Advice – Which Engine Oils to Use


Images: Courtesy & Copyright Dave Marion

Written by David H. Marion. Time for an oil change in your airplane? Here’s some simple, practical aircraft maintenance advice for a change of pace. Motor oil is cheap insurance for your engine – keep it serviced properly and change the oil and filter at least as often as recommended – if not even more often. The general rules of thumb are to change the oil and filter, if it is a “spin-on” disposable paper element type of filter that is, every 50 hours of operation. Not so common any more, fortunately, are the old permanent brass screen filters on Continentals or steel mesh filters on Lycomings. With them, it was a good idea to change the oil and clean and re-install the filter every 25 hours of operation.

That being said, don’t be “cheap” in your choice of which oil to use. I will go out on a limb and say that I personally do not like Phillips X/C 20W-50 engine oil, but mostly because I don’t like any “multi-grade” oils for aviation use. Just to be fair, I also don’t like Aeroshell 15W-50 oil either. And I have known many owners who use the Phillips oil only because it is generally cheaper than other alternatives. In my opinion, that is not a valid criterion on which to base such an important decision.

Furthermore, I especially do not like using any type or brand of multi-grade oil in “big bore” Continental engines (520 and 550 CID) and even more so when it comes to any kind or size of turbocharged engines. My sentiment in that regard is based on over 10 years of specifically relevant experience working for an engine overhaul facility and seeing firsthand the differences in internal wear and damage to a variety of many, different engines.

Aircraft Maintenance Shops have to keep a widely varying supply of different oils to suit the needs of their customers

20 years ago or so, Richard Collins of FLYING magazine mentioned in one of his columns that his Continental TSIO-520 series engine in his beloved Cessna P210 aircraft never seemed to make it all the way to its TBO (Time Between Overhauls) before conditions forced him to have it overhauled again. I wrote to him and asked him which oil he regularly used in it and he answered that it was always the Phillips 20W-50. I told him of my experience and that of the engine shop for which I was working at the time and politely suggested that he start using “straight weight” Aeroshell W100 oil instead. He seemingly arrogantly rejected my suggestion out of hand and claimed that he was perfectly satisfied with the Phillips oil – even contrary to his own complaint that his engine never made it to TBO! Apparently he failed to see the relationship between them – or the value of listening to someone else who just might have had wider and more varied and relevant experience with the subject at hand.

In another, much more recent case, a regular customer who owns and flies a turbocharged, Lycoming IO-540 powered* Piper PA-23-250(6-place) Aztec recently provided positive feedback on that same issue as well. He used to run the “cheaper” Phillips 20W-50 oil in his engines – until we “accidentally” changed it out for Aeroshell W100 during the routine Annual Inspection on his airplane last year. Since then, he has admitted that his engines are actually running cooler now too and he is happy to stay with the Aeroshell from now on.

(*That’s not a typo – the early Turbo Aztecs had Lycoming IO-540-J4A5 engines that were nominally normally-aspirated but then were modified during installation on the aircraft and “turbo-normalized” using an aftermarket turbo system. They were and are not “boosted” with any higher manifold pressure and extra horsepower, but are configured rather to be able to maintain what power they develop at sea level to higher altitudes through the use of the turbo system. Later model Piper Turbo Aztecs by comparison were built with Lycoming TIO-540-C1A series engines, but they too were not boosted over the normally-aspirated models’ 250 hp.)

It’s not directly related to my subject of aviation oil usage, but in regard to Richard Collins and turbocharged Lycoming engines, I’d like to relate a tangential story as a little aside. Back many years ago as well, I also wrote to Mr. Collins to correct him on something else about which I believed he was mistaken. Around that same time (in the mid-1990’s) Mooney Aircraft had come out with their model “TLS” or M20M series. The “TLS” stood for Turbo Lycoming Sabre and it was initially equipped with a 270 hp. TIO-540-AF1A engine.

All of the pilot/writers staffing the various industry journals, including Mr. Collins at FLYING, marveled at how wonderful the engine in the TLS must be since, as they incorrectly assumed, it was “de-rated” from the 350 hp versions of the Lycoming TIO-540 engines such as the –J2BD series used on the Piper PA-31-350 Navajo Chieftain. They reasoned further incorrectly that as such, the -AF1A series would be accordingly less stressed and particularly robust and durable. They were wrong.

This is a 325 hp Lycoming TIO-540-F2BD on a Piper PA-31-325 Navajo C/R – it represents the higher horsepower “angle-valve” variants of the TIO-540 series

If you take the time to read the details shown in FAA Type Certificate no. E14EA for the whole of Lycoming’s TIO-540 series, the ONLY three things that a -J2BD engine and an -AF1A engine have in common are the facts that they are covered by the same TC, they both have 541.5 cubic inches of displacement, and they both are painted with the same gray paint. Everything else about them is completely different. First of all, just as with their normally aspirated cousins, the ones with the “parallel valve” cylinder heads are always rated for less power than the ones with the “angle-valve” cylinder heads. On the basis of the different cylinders types, they each have completely different cases, crankshafts, camshafts, etc.

The TIO-540-AF1Aparallel valve” engines are in fact “boosted”, harder working variants derived directly from the 250 hp. TIO-540-C1A engines used on the aforementioned, late-model Piper Turbo Aztecs. Whereas they are all “red-lined” at only 2,575 rpm (although many normally aspirated 540 series engines can go up to 2,700 rpm) the manifold pressure of the –C1A is limited to 33.0 in-Hg. at sea level and 34.0 inches at 15,000 ft. The similar “parallel valve–AF1A and the much different “angle valve-J2BD by comparison are rated for 35.0 and 43.0 inches respectively at sea level and 36.0 and 46.0 inches respectively at 15,000 feet. That last set of numbers represents a quite significant 5 psi difference in boost on those engines.

The bottom line is that all of the pilot-slash-magazine editors who made that same mistake had no clue as to what they were talking about and they were all dead wrong. Not so long afterward, problems in service began to arise with the Mooney TLS and its TIO-540-AF1A engines. Lycoming came out with Service Instruction SI 1479 requiring the installation of external oil lines to supplement the oil flow to the cylinder heads and the exhaust valve guides in particular to prevent excessive wear and damage – just like the ones used on the 2,900 rpm, 380 hp TIO-541 series engines used on the Beech Duke. So modified, the Mooney’s engine was re-designated as a model –AF1B and the airplane itself re-designated as a TLSBravo.”

So, in moving on with my advice on engine oils, another reason many aircraft owners think that they ought to be using multi-grade oils in their aircraft engines is cold weather starting. However, in my professional opinion, if you keep your airplane in a hangar and/or have an oil sump, pad-type pre-heater installed or use an external pre-heater before starting your engine(s) in extreme cold weather, then it is not necessary to use any kind of multi-grade oil. In summer or hot weather, I tell customers who are willing to listen that they ought to use straight 100 grade oil and in winter or other consistently cold weather to use straight 80 grade – equivalent to SAE grades 50 and 40 respectively.

This is a 250 hp Lycoming IO-540-J4A5 engine on a 1966 Turbo Aztec “C”. It is modified with an aftermarket turbocharger installation and it represents a typical “parallel valve” 540 series engine

In case you were not already aware of it, SAE stands for Society of Automotive Engineers, which is not exactly a primarily “aviation” standards organization. Plus, the “W” in 10W-30 or 20W-50 for two examples stands for “Winter” not “weight” as is sometimes thought – i.e. that number is its cold weather viscosity rating or effective grade. If you are not flying your aircraft often enough to need to change the oil regularly between your local hot or Summer and cold or Winter seasons, then you are simply not flying it often enough period!

Furthermore, in case any of you are interested in from where those numbers actually come or are derived, the viscosity of oil is typically measured by a specific piece of test equipment called a Saybolt Vicscometer. The test sample is dripped through the tester and the time it takes for the standard sample to go through the tester is measured in seconds. The reading is quantified in Saybolt seconds and they are typically on the order of 1,000 to 1,100 seconds. For purposes of commercial grading, the first 1000 is just dropped outright. Because of that, a reading of 1,100 Saybolt seconds for example becomes Aviation grade 100 and for SAE grading, that same number is then just divided by 2 to produce so-called “50 weight.”

The other thing that you ought to know about motor oil is that basic (i.e. “straight weight”) oil does not “break down” in terms of viscosity – not ever. The viscosity of Aeroshell 100 will always be that under any and all conditions – it’s only problem is that it gets dirty that is why you need to change it! In terms of multi-grade oils however, it is the additives which cause its viscosity to vary and behave normally backwards from other liquids (which more typically get thicker when they are cold and thinner when they are hot.) And it is those additives that break down and lead to “viscosity breakdown” – and in turn cause a lot of problems in aircraft engines.

Speaking of oil sump, pad-type pre-heaters as I was a bit earlier – I only recently learned from an even more experienced mechanic, the current director of maintenance at the engine shop where I used to work as a matter of fact, that they recommend not using the top, case-mounted heater pads that come with some of those engine pre-heater kits these days. Similarly based on their extensive experience with engines coming in for overhaul and being torn down for inspection, they have seen more problems with those engines with the extra heater pads on the tops of their cases than with the ones with only the oil sump pads near the bottom.

It’s not exactly a long-term or scientific study, but that engine overhaul shop speculates that the upper pads cause more evaporation and eventually condensation of moisture inside the case – so on a Continental engine they see more rust and corrosion on the crankshaft and on a Lycoming the same on the camshaft. I can’t say whether or not the pre-heater manufacturers will even acknowledge this issue, much less stop making and selling the extra, top-case pads, but my advice (based on the experience and reputation for quality of the engine shop that I trust) is that if one is included in your new heater pad kit, throw it away and do not use it.

You have to look closely to see that there are only 2 cases of Phillips X/C 20W-50 in our stock. Most everything else is Aeroshell brand products because that’s what we prefer based on literally decades of experience

There is one more thing about oils that I learned only just recently, but which is contrary to the some of the conventional wisdom regarding which types of oil to use and when. After installing a freshly overhauled Lycoming TIO-540 series engine on a Piper PA-31 Navajo, I was told by the engine overhaul shop that Lycoming specifically recommends against using plain “mineral” oil for initial break-in of any of their turbocharged engines because the extreme heat of the turbocharger can “coke” the oil quickly. In other words, that extreme heat can cook or burn the oil and cause a build-up of combustion byproducts which in turn can clog the lubrication galleries in the turbocharger’s bearing housing in particular or other parts of the engine in general.

Accordingly, Lycoming specifically recommends instead actually using “detergent” or “Ashless Dispersant” (AD) type oil for the initial break-in period even though that is contrary to the conventional wisdom with which most pilots and mechanics are probably familiar. The technical reference data for that can be found in Lycoming Service Instruction no. 1014 Rev. M dated May 22, 1995 – so it’s not exactly a new thing, but this was the first that I had heard of it. (Even as a mechanic with 30 years of experience, it’s good to keep learning new things – there is so much out there to know!)

That too is another reason to pay more attention to those sometimes obscure or ignored service bulletins and service instructions that some so-called experts will tell you are never “mandatory.” That may be true from a legal or FAA perspective, but guess what? What you don’t know or don’t do in terms of maintaining your aircraft can in fact end up hurting you or your aircraft! It also goes to show you that you can learn quite a bit about your engine (and of course your aircraft too) by simply reading the supplemental service documents and applicable Type Certificates.

Dave Marion is the Technical Content Editor at As A&P and IA with 30 years of experience in aircraft maintenance, he is also a Commercial Pilot with Airplane, Single & Multi-Engine, and Instrument ratings. He has a BA from Colgate University in 1984 and also graduated cum laude from Embry-Riddle Aeronautical University (DAB) with a BS in Aviation Technology in 1990. He can be reached along with all of the editors via E-Mail:

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