Which oil is really best for your expensive aircraft engine? Or are they all really just the same stuff packaged in different colored bottles and distinguished only by brand names or by advertising claims that may or may not reflect reality?
Frankly, answering these questions is one of the most difficult and expensive research projects The Aviation Consumer has undertaken. Oils for piston aircraft engines serve multiple purposes, including lubricating, cooling, capturing combustion by-products and keeping them in suspension and preventing corrosion when the engine is idle.
All of these jobs are interrelated so even though an oil may be good at reducing wear, if its weak on preventing corrosion, it may be a poor choice if the engine isn’t flown frequently. On the other hand, if a turbocharged engine is flown hard and the oil changed infrequently, all the corrosion protection in the world may not protect it against premature wear.
With this in mind, we embarked upon a round of laboratory and informal field tests to find out which aviation engine oils really do meet the advertising claims. In this article, we’ll describe only anti-wear qualities. we’ll examine anti-corrosion issues next month. Our tests didnt reveal a definitive winner from an anti-wear standpoint. But in our estimation, there may be enough differences in engine oils to make it worth picking one over another.
When it comes to protecting an engine from wear across a wide range of operational situations, were most impressed with AeroShells W100Plus and 15W50, two of the most widely available aviation oils on the market. We have some reservations about these oils with regard to corrosion protection, which we will address later, but overall, our view is that these two products perform best when all the variables are taken into account.
The other popular oils we tested were Shells W100, Phillips X/C, and Exxons much-talked-about Elite. Interestingly, when pushed beyond the limits the typical GA engine is likely to encounter, these oils finished significantly behind AeroShell W100Plus and 15W50 in anti-wear properties in the lab tests we performed.
Thats not to say, however, that the other oils arent credible products, since they all pass FAA, SAE, OEM and MilSpec requirements. Still, we think that under the most adverse conditions, Shell products containing anti-wear additives (W100Plus and 15W50) enjoy an edge in wear protection.
Now, on to the tests. But first some background in aviation oil technology, which will help you understand how wear additives work.
Oil Basics
Oil may be oil but the fact is, its the additive package that distinguishes one brand from another. Additives can significantly improve various properties of oil, including wear protection, corrosion and oxidation protection, viscosity index, friction and cleanliness.
Unrefined crude oil contains a chemical cornucopia of molecules, each having different physical and chemical characteristics. These so-called base stocks can be refined to varying levels of purity and performance.
They are first distilled into the various weights or viscosity grades. They are then solvent refined to remove wax and to improve their low temperature properties, mainly pourability. This base stock is now referred to as a group I base stock and is the most commonly used for general engine oils.
These stocks can then be further refined by a process called hydrogen finishing to improve their stability and cleanliness. Hydrofined base stocks are classified as group II and are currently much in favor for passenger car motor oils.
Mineral base stocks can be further manipulated at the molecular level in a process called catalytic isomerization, which transforms them into a high-viscosity index group III base stock designed to compete with what are known as synthetic base stocks.
Synthetic oils are made up of man-made molecules, which are synthesized to give specific properties. These properties can include thermal stability and improved viscosity index. Polyalphaolefin (PAO) is the most common synthetic base stock used in motor oils. Its composed of molecules synthesized from ethylene gas and is made in a wide variety of viscosities.
The problem with synthetic PAO, however, is that it has poor solvency characteristics which are exacerbated when its used in engines burning 100LL or any leaded fuel, for that matter.
Lead, which forms salts and oxides as a combustion by-product, can form lead sludge if its not kept in suspension and drained when the oil is changed.
This is one reason why Mobil may have run into problems with its Mobil AV1 aviation oil, which was withdrawn from the market. AV1 was 100 percent PAO but oil chemists tell us that no one has produced a good additive package for PAO base stock that will improve solvency and lead scavenging, although Mobil would certainly argue that it had an adequate additive package for AV1.
You may recall that Mobil reached settlements with a number of aircraft owners who claimed their engines had been damaged by AV1, mainly due to excessive lead sludge and deposits. And therein lies the risk for any manufacturer introducing a new oil. (Note that Mobils automotive variant, Mobil 1, is recommended only in engines using unleaded fuels which, these days, is virtually everything on the road.)
One solution to PAOs weak solvency characteristics is to blend synthetic base stock with mineral oil and thus get the best of both worlds. This is what both Exxon and Shell do in their 15W50 oils while Phillips uses the tried-and-true mineral oil base stock in their multi-grade Cross Country X/C.
All current ashless dispersant engine oils for aircraft use contain mineral base stocks along with dispersant and anti-oxidant additives. Multi-viscosity oils have a viscosity modifier, which is a polymer designed to reduce the thinning of oil with increasing temperature.
To make a semi-synthetic blend, the refiner simply adds some percentage of PAO to any oil. The semi-synthetic varieties such as Shell 15W50 and Elite are just this, a blend. Generally, semi-synthetic GA oils contain from 25 to 45 percent PAO. Both also contain proprietary anti-wear and anti-corrosion additives unique to each brand and thats the foundation upon which the advertising claims are often built.
Additives Make the Oil
Under normal circumstances, straight mineral oil is good enough to get your engine to TBO. But normal is defined as an engine thats run frequently, has regular oil changes, is started in below-freezing temperatures only when pre-heated and is never over-boosted or exposed to excessive temperatures and/or dirty environments. In the real world, few privately flown aircraft are operated that way so oil manufacturers spend lots of money to tweak their additive packages to both prevent corrosion and metal-to-metal wear.
There are tradeoffs between effective protection, possible product liability uncertainties of new formulations and cost. Not all GA oils contain anti-corrosion and/or anti-wear additives. Shells straight weight W100 and Phillips X/C are examples of mineral oils with ashless dispersant additives but without any anti-wear or anti-corrosion additives.
Most anti-wear additives work by decomposing and reacting with the surface of the metal being lubricated, laying down sacrificial films that are continuously replenished as they wear away. Others, such as esters and other organics, are formulated to have molecules that bond to the surface and resist being pushed off under load.
Ashless additives, such as the triaryl phosphate ester functional fluids, tricresyl phosphate (TCP), triphenyl phosphate (TPP) have historically been utilized as anti-wear, plasticizers and flame-retardant additives in many applications. They are found in turbine engine oils where at 400 to 500 degrees F, they decompose and form phosphate films on the bearing surfaces, reducing wear.
These additives have not demonstrated good performance in automotive applications and have never found widespread use. Automotive gasoline and diesel engines depend on the use of zinc diakyldithiophosphate zinc for anti-wear protection.
When they are used in GA oils, anti-wear additives are historically phosphate esters. These work by coating parts with a sacrificial layer of molecules that prevent metal-to-metal contact. So called extreme pressure additives are used in some general purpose application oils but not in aviation oils.
Generally speaking, Continental and Lycoming engines don’t have operating pressures in excess of 7000 to 8000 PSI at any metal-to-metal surface. But in other applications, such as automotive gear boxes, manual transmissions and marine outdrives, extreme pressures are more common.
These applications require oils with specialized extreme pressure (EP) additives to keep the surfaces from scuffing and wearing. By far, the overwhelming EP anti-wear additive in motor oil is zinc dialkyldithiophosphate (ZDDP). There are many varieties of ZDDP but they all form sacrificial anti-wear films on rubbing surfaces as the molecules decompose.
Theyre activated in working environments which develop pressures greater than 25,000 PSI. If not activated, these additives don’t help much for wear but are still good antioxidants.
So why are we mentioning EP additives if they don’t apply to aviation oils? Because of how lab oil testing works. Most laboratory testing of anti-wear compounds use accelerated tests and generally use pressures greater than typically found in engines and thus can border on extreme pressures. In our testing, we were careful to limit our pressures to 15,000 to 30,000 PSI to test for low-load, anti-wear characteristics and to steer clear of the classic EP regime.
Small aircraft engines operate with significantly lower pressures between rubbing surfaces than other types of powerplants, which is why they can run on straight mineral oil formulated with no anti-wear additives at all.
After all, these engines were designed years ago at a time when there were no anti-wear additives and they ran just fine with acceptable durability.
So the question becomes; do anti-wear additives really do much for your engine? Our view is yes, they do, especially in abnormal situations such as over-boosting, cold starts, infrequent use, forgotten oil changes and other abuse. In these circumstances, youd rather have the additives than not. Yes, one additive may have a downside but its advantages may offset its disadvantage in most cases. More on that in a moment.
Before explaining the test results, we need to cover another key point: corrosion protection additives. We think its critical to consider both anti-wear and anti-corrosion properties when evaluating an oil. Depending on how the engine is operated, an oils ability to prevent corrosion may be its most important property.
How The Oils Fared
Our lab tests were designed to test oil anti-wear properties in simulated wear conditions just beyond what straight mineral oil will handle by itself, sans specific anti-wear additives. The sidebar on page 6 explains how the tests were done. These tests were overseen for us by Ed Kollin, of Lubrication Science Labs, in Scotch Plains, New Jersey. Kollin is an independent consultant with years of experience in the field, including a stint at a major oil company. He has developed his own oil additive which, although not yet marketed, was included in some of the testing strictly for benchmark purposes. we’ll report on additives in another issue.
As for this round of testing, basically, a machine called a V-block tester is used to accurately apply increasing pressure to a steel rod, the weights and dimensions of which are precisely known. Once the test is completed, the part can be accurately weighed to determine how much metal was lost to wear. Although it doesnt necessarily mimic real-world engine conditions, the V-block test does represent a standard test method.
Again, we wish to be clear: Our tests clearly exceed the requirements of MilSpec L-22851 and SAE J-1899, which are the basis for FAA approval. All of the oils tested meet those standards. These tests don’t reveal or suggest that any of the oils we tried arent suitable for routine use. But they do show that some oils arent as good as others in the extreme corners of the operational envelope.
Exxons new Elite, according to our tests, is one of the latter. Elite is a semi-synthetic, multi-viscosity blend which failed immediately upon reaching the assigned load of 350 pounds on the test rig. These pressures corresponded to something like 20,000 PSI metal-to-metal working pressures.
This test suggests that the anti-wear chemistry used in Elite isn’t as effective as the Shell oils in the ranges we tested in this protocol. Again, these ranges may be outside what you would expect to encounter in normal operation but its also true that you could encounter such conditions. Surprisingly, Elite performed not significantly better than straight-weight mineral base stock, even though it has anti-wear additives.
Shells W100 Plus and 15W50 semi-synthetic, both of which contain Shells anti-wear additives and meet Lycomings specs, performed much more impressively. Shells AeroShell W100 Plus and multi-grade products showed superior anti-wear properties over all other oils tested except the ones with the experimental GL-1000 additive package. we’ll have more on that in the next article.
Those Shell products that don’t contain anti-wear additives-specifically the W100 straight weight oil-also failed the anti-wear test, as we would expect. Phillips multi-grade X/C doesnt meet the Lycoming additive specs requiring an anti-scuff additive and, in fact, doesnt contain any specific anti-wear chemistry at all. As expected, it failed our test at 75 seconds at 350-pounds loading, even at a much lower oil temp (150 degrees F) than the others.
We contacted all of the oil companies for their comments on our test results. Shell and Exxon chose not to comment but did answer our technical questions. Exxon was in possession of the test data for more than a month, it declined to comment but told us it may have a response in the future, which we have agreed to publish. Phillips said our test results may or may not have much to do with how oil performs in the field notes that X/C exceeds FAA, SAE and MilSpec requirements. Phillips insists that field experience shows that X/C has excellent anti-wear properties.
Conclusion
So what does it all mean? Is Shell the slam-dunk winner over Exxon Elite? It depends. All of the oils tested will perform satisfactorily when used in accordance with both engine and oil company recommendations in situations where little or no abuse is encountered.
In our estimation, Shells W100Plus and 15W50 demonstrate better anti-wear performance than the other oils. Understandably, Phillips X/C and Shells W100 both failed our anti-wear tests when the testing pressures exceeded those typically found in GA engines. But these oils have no anti-wear additives so this wasnt especially surprising.
What was interesting was that Elite didnt do much better and it too failed, even though it contains what Exxon claims is a sophisticated anti-wear package.
Shells W100Plus, 15W50 and Elite all meet Lycomings AD80-0403 and contain a Lycoming approved anti-scuff additive similar to Lycs part LW 16702. However, in achieving this level of anti-wear performance, Shells W100Plus and 15W50 oil may have a weakness in that their specific phosphate ester anti-wear additive is not hydrolytically stable. That means that it decomposes in the presence of water. The decomposition products are corrosive and tend to attack soft metals such as copper, lead and tin.
To some extent, this accounts for the copper leaching problem Shell has faced in years past with these oils and which has been reported in the field. Shell told us they have since added an additional copper passivator to combat this problem and weve noted that field oil analysis supports Shells claims.
Maintaining proper oil temps (above 210 degrees F) as we’ll as frequent oil changes may minimize any corrosion problems that the phosphate ester anti-wear additive may introduce.
Even though Elite didnt fare we’ll when compared to Shells products in our lab wear testing, in the real world of infrequent oil changes and short flights, Elite may still be a good choice because it has impressive anti-corrosion characteristics, which we will be reporting on in a future issue. Further, it doesnt have the type of phosphate esters found in Shell, thus it has no moisture degradation problems.
So, which would we use in our engine? It depends. If the airplane is flown regularly on long flights and the oil was changed frequently, we would choose either Shell W100Plus or 15W50 semi-synthetic. It has the best anti-wear characteristics, possibly at the expense of some moisture sensitivity. But frequent oil changes can address that shortcoming.
If we only flew around the patch every month or two and only changed the oil every six months or so, then Elite would be an attractive choice, even though its not as impressive as Shell from the anti-wear point of view.
In the next article in this series, we’ll address corrosion protection in detail. For most owners, its the far bigger enemy of engine longevity than metal-to-metal wear is.
Also With This Article
Click here to view “Checklist.”
Click here to view “Whats In A Label” and “Oil Wear Test Results.”
Click here to view “How the Testing Was Done.”
-by Coy Jacob
Coy Jacob is an Aviation Consumer contributing editor. He operates the Mooney Mart complex in Venice, Florida.
