The Savvy Aviator #52: Thinking About Oil Changes


Last July, I made my annual pilgrimage to EAA AirVenture in Oshkosh, Wis., with a detour to Cadillac, Mich. The purpose of the detour was to participate in a pre-AirVenture floatplane weekend organized by AVweb columnist Rick Durden (The Pilot’s Lounge). The Cadillac Lake shindig is an informal affair where a bunch of pilots spend two days flying Super Cubs on floats and renewing old friendships. It’s relaxing and enjoyable and exactly what I need before the pressure-cooker week of AirVenture.My 3,500-nm trip from home base (Santa Maria, Calif.) to Cadillac, Oshkosh, and return involves 19 hours of flying time in my Cessna T310R. While preparing for the trip, I checked my maintenance records and found that it had been 39 hours since I changed the oil. My normal oil-change interval is 50 hours, so it seemed obvious that I’d better change the oil and filter before leaving on the trip, 11 hours early. Otherwise, by the time I got back from Oshkosh, my oil would be 58 hours old.


As I thought about this a bit more, I considered that once I got back from Oshkosh on July 31, my schedule didn’t call for another trip until mid-September. Consequently, the airplane would likely be sitting idle for a month and a half. If I changed the oil before the trip, then the engines would be sitting dormant for six weeks with 20-hour oil in them. On the other hand, if I delayed the oil change until I got back from Oshkosh, then the engines would be full of fresh, clean oil during the dormant period. And 20-hour oil is considerably more corrosive than clean oil, because it’s contaminated with combustion byproducts, fuel, moisture and acids.Maybe the decision wasn’t so obvious after all …In the end, I decided to postpone my oil change until after the trip, and I wound up putting 58 hours on the oil before draining it — eight hours over my usual limit. I started the oil draining immediately upon my return home, and went down to the hangar the next day to finish the job by changing the oil filters, cutting open the old ones and servicing the engines with fresh Aeroshell W100. Then I took the airplane up for 20 minutes to make sure that all the internal engine parts were thoroughly coated with fresh oil.While at AirVenture 2007, I ran into Ed Kollin at the ASL Camguard booth. Ed is a lubrication research chemist by trade — formerly head of the Exxon Engine Research Laboratory, Advanced Fuels and Lubricant Group — and knows more about the chemistry of engine oil than anyone I know. I described my predicament to Ed and asked him whether he thought I made the correct decision to change the oil after the trip rather than before. Ed agreed that after was better. (He added that perhaps an even better choice would have been to do two oil changes, one before the trip and one immediately afterward.) Ed was quite emphatic that letting the engines sit unflown for a month while full of 20-hour-old oil was definitely a bad idea, because 20-hour-old oil is pretty nasty stuff.

Why We Change The Oil

Contrary to what you may have heard, we don’t change the oil because the oil breaks down in service and its lubricating qualities degrade. The fact is that conventional, petroleum-based oils retain their lubricating properties for a very long time and synthetic oils retain them nearly forever.Consider, for example, that Ford Motor Company now recommends a 7,500-mile oil-change interval for most of its cars and trucks, which is the equivalent of 150 to 200 hours. In fact, oil analysis studies have shown that a synthetic automotive oil like Mobil 1 or Amsoil can go 18,000 miles without appreciable degradation, and that’s the equivalent of 400-500 hours.No, the reason we change oil in our aircraft engines every 25 to 50 hours is not because it breaks down. It’s because it gets contaminated after 25 to 50 hours in an aircraft engine. In fact, it gets downright filthy and nasty.


Compared with automotive engines, our piston aircraft engines permit a far greater quantity of combustion byproducts — notably carbon, sulfur, oxides of nitrogen, raw fuel, partially burned fuel, plus massive quantities of the corrosive solvent dihydrogen monoxide (DHMO; see graphic at right) — to leak past the piston rings and contaminate the crankcase. This yucky stuff is collectively referred to as “blow-by” and it’s quite corrosive and harmful when it builds up in the oil and comes in contact with expensive, bottom-end engine parts like crankshafts, camshafts, lifters and gears.To make matters worse, avgas is heavily laced with the octane improver tetraethyl lead (TEL), which also does nasty things when it blows by the rings and gets into the crankcase. (If you’re as old as I am, you may recall that back before mogas was unleaded, Ford’s recommended oil-change interval was 3,000 miles instead of 7,500 miles.)So one of the most important reasons that we need to change the oil regularly in our Continentals and Lycomings is to get rid of these blow-by contaminants before they build up to levels that are harmful to the engine’s health.

How Often To Change

Another reason we need to change the oil regularly — arguably even more important than disposing of contaminants — is to replenish the oil’s additive package, particularly its acid neutralizers. When sulfur and oxides of nitrogen mix with DHMO, they form sulfuric acid and nitric acid. If you remember these dangerous corrosives from your high school chemistry class, then you’ll appreciate why you definitely don’t want them attacking your expensive engine parts.To prevent such acid attack, aviation oils are blended with acid-neutralizer additives. These are alkaline substances that neutralize these acids, much as we might use baking soda to neutralize battery acid. These acid neutralizers are consumed by the process of neutralizing acids, so it’s imperative that we replenish them before they get used up to an extent that might jeopardize our hardware. Of course, the way we replenish them is to change the oil.How can we tell when the acid neutralizers in the oil have been used up? It turns out that there’s a laboratory test that measures the level of unneutralized acid remaining in the oil. This is known as the “total acid number” or TAN test. Some oil-analysis firms can perform this test on your oil samples. However, it’s not routinely done as part of the normal oil-analysis report, so you need to specially request a TAN test when you send in your oil sample (and be prepared to pay extra for it).Most owners don’t bother with the hassle and expense of TAN testing and simply change their oil at a conservative interval that’s guaranteed to get the junk out and fresh additives in before anything untoward is likely to occur. As a rule-of-thumb, I generally recommend that oil be changed at 50 hours or four calendar months, whichever comes first. The effect of this rule is that operators who fly at least 150 hours a year can go 50 hours between oil changes, but operators who fly less will use a proportionately reduced oil-change interval.The forgoing recommendation assumes that the aircraft has a full-flow (spin-on) oil filter installed, that it operates primarily from paved runways and that it has decent compressions and oil consumption. Engines that have only an oil screen (no filter) should have the oil changed every 25 hours. Engines that operate in dirty or dusty conditions and ones that have high oil consumption or weak compression should have more frequent oil changes.Another important result that appears in the normal oil analysis report provided by some labs — including the one I use, Blackstone Laboratories in Ft. Wayne, Ind. — is the “insolubles” test. This test is performed by placing the oil sample in a centrifuge to separate out all solids and liquids in the sample that are not oil-soluble.Virgin oil normally contains no insolubles. The insolubles found in drained engine oil come from three sources: (1) oxidized oil that breaks down due to excessive heat; (2) contaminants from blow-by of combustion byproducts; and (3) particulate contamination caused by poor oil filtration. If your oil analysis report reveals above-normal insolubles, it might be indicative of an engine problem — high oil temperature, excessive blow-by, inadequate filtration — and almost certainly means you should be changing your oil more frequently.By the way, did I mention that I’m a huge fan of laboratory oil analysis? I use it religiously, recommend it strongly to all piston aircraft owners and believe that it’s one of the most important tools we have — along with oil-filter inspection and borescope inspection — for monitoring the condition of our engines and determining when maintenance is necessary.

Choosing Oil And Additives

Much has been written about the pros and cons of various types and brands of engine oil. Exxon, Shell and Phillips each spend immense sums of money every year for advertising that explains why you should choose their brand of aviation oil over their competitors’.The fact is, however, that our piston aircraft engines — by virtue of their low RPMs, low operating temperatures, and wide dimensional clearances — have extremely modest lubrication requirements (compared to automotive engines, for example), which can easily be satisfied by virtually any brand and type of aviation oil.Blackstone Labs maintains an extensive database of wear metals produced by almost all types of piston aircraft engines. They performed an analysis that compared the wear metals generated by 571 Lycoming IO-360 engines using four types of oil: Aeroshell W100, Aeroshell 15W-50, Exxon Elite 20W-50, and Phillips 20W-50. They found no significant difference in wear metals between the four types of oil.Another important oil-related issue is preventing corrosion (rust) when an engine is dormant for a period of weeks or months — something that seldom happens to “working airplanes” (charter, rental, flight school, flying club, etc.), but is unfortunately common among owner-flown airplanes. Rust is the #1 reason that engines fail to make manufacturer’s recommended TBO, so preventing it is extremely important.


This is where controversy abounds. Both Aeroshell and Exxon go to great lengths in their advertisements to proclaim the superior corrosion-preventive capabilities of their flagship multigrade oils (Aeroshell 15W-50 and Exxon Elite 20W-50) and offer all sorts of laboratory test results to substantiate their claims. Similarly, an independent study performed by and published in The Aviation Consumer confirmed that these two oils were superior at preventing rust on steel plates in a salt-water-cabinet test.However, I’ve discussed this issue with many experienced engine builders at most of the leading aircraft-engine overhaul shops and they have a very different view. Almost without exception, the engine builders recommend that owner-flown aircraft that fly irregularly (as so many do) and are therefore at high risk for rust damage are better off using a thick, single-weight oil (such as Aeroshell W100 or W100 Plus) and avoiding the use of multigrade oils except when absolutely necessary due to cold-weather operations. The logic is that during extended periods of disuse, you want the ferrous-metal parts of the engine protected by the thickest, gooiest stuff possible. (At room temperature, single-weight oils like Aeroshell W100 have the consistency of blackstrap molasses, while multigrades like Aeroshell 15W-50 are more like Aunt Jemima Light Syrup.)My own personal experience agrees with this: I have investigated many cases of premature cam and lifter distress (generally caused by corrosion during periods of disuse) and without exception they all involved engines operating on Aeroshell 15W-50 multigrade. For the past 40 years, I’ve used nothing but single-weight Aeroshell W100 in my aircraft, and have enjoyed engine longevity that is nothing short of phenomenal.A related subject that often comes up involves the use of aftermarket additives — Microlon, AvBlend, Marvel Mystery Oil, etc. As a general rule, I’ve never been a fan of such additives. I’ve witnessed independent objective tests of both Microlon and AvBlend that strongly suggested that they make no measurable difference. I’m pretty sure that AvBlend at least does no harm, but I’m not so sure that’s true of Microlon. (At his technical forums at Oshkosh, my colleague John Frank — executive director of the Cessna Pilots Association and a world-class maintenance expert — has often been heard to exclaim, “I wouldn’t put Microlon in my lawnmower, much less my aircraft engine!”)Marvel Mystery Oil — which is basically a thin, petroleum-based solvent to which red food coloring and perfume has been added — does seem to have some value in freeing up sticky hydraulic-valve lifters when added to the oil and it almost certainly does no harm. However, I wouldn’t recommend using it on a regular ongoing basis (in the absence of audible valve-lifter clatter), and I certainly would discourage anyone from adding it to avgas (as opposed to oil).I do try to be open-minded about these things, however. After talking at length with Ed at Oshkosh this year and studying all the available literature, I must confess that I flew home with four pints of Ed’s new oil additive ASL Camguard in the baggage compartment of my 1979 Cessna T310R, and I poured two of them into my two 800-hour-past-TBO Continental TSIO-520-BBs at the post-Oshkosh oil change. Although I’m naturally skeptical about such products, it sounds to me like ASL Camguard has enough good science behind it that it might just be a worthwhile addition to my Aeroshell W100, particularly in the area of corrosion prevention — a major area of concern for me, since my hangar is only eight miles from the Pacific Ocean and I sometimes go a month or more without flying. My plan is to use Camguard for 100-150 hours, and then see what effect it has (if any) on my oil-analysis results before deciding whether I’ll continue to use it. Either way, I’ll write up my findings — film at eleven.See you next month.

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