Hangar or not, the only thing between you and enough corrosion to turn your engine to junk is the oil you use. But which oil does it best? And are single-weight oils better at corrosion prevention than multi-weights?
We set out to find out in some recent informal corrosion testing, squaring off the popular engine oils in a free-for-all rust prevention round-up.
Heres the background: When ExxonMobil introduced its new Elite aviation engine oil last spring, the company made such impressive claims for this new oil that the market was jolted out of a long snooze.
In our August 2000 issue, we examined Elite oil and reported on the results of some informal corrosion testing in which Elite did remarkably well.
But a number of readers asked us for more, specifically a look at several straight-weight oils, which we overlooked in the previous tests. Here then, is a second round of testing.
How and What We Tested
For our first round of tests, we asked a couple of oil company representatives for a suitable means of quick testing corrosion resistance qualities of aircraft engine oils.
A valid test, they agreed, was to spray steel wool pads treated with oil with a brine mixture and observe the development of corrosion. Furthermore, to add some real-world data, we also corrosion-tested some Lycoming cams.
A disclaimer: Were not implying that this test directly simulates the development of corrosion on the bright machined surfaces of engine cams, cranks and cylinders, since those parts arent made of steel wool. However, it more than satisfies our curiosity and allows us to draw some general if not definitive conclusions.
This time, we examined ExxonMobil Elite, Shells semi-synthetic 15W-50 AeroShell, Shells new W100 Plus straight-weight and its older W100 and Phillips XC, a 20W50 multi-weight.
For our test, we obtained several unserviceable Lycoming cams, washed them thoroughly to remove any old oil and then soaked them thoroughly in all four types of oil. In addition, we treated steel wool pads in the same way, then sprayed sea water on them on a regular basis for several days.
During the test, we examined each sample for signs of rust and, sure enough, it showed up fairly quickly. As a control, we treated one pad with Corrosion-X, a highly regarded anti-corrosion compound widely used on airframes but not in engines.
Exposed to the open air and assaulted with brine, we werent surprised to see corrosion develop quickly, especially on the steel wool, which will rust on a humid day if given the chance.
Within a day or two, the cams treated with Phillips XC and Shells older W100 showed tiny specks of corrosion on machined surfaces. According to both Phillips and Shell, neither of these oils are blended with specific anti-corrosion additives and it shows.
However, Shells AeroShell 15W50 and its new W100 Plus do have anti-corrosion additives and after two days, these samples showed noticeably less corrosion, although they werent entirely corrosion free. The cam treated with Elite showed the least rust of all, although again, it wasnt entirely rust free. Interestingly, even when rust isnt visible on the surface of the metal, a close examination with a magnifying glass will reveal tiny specks blooming here and there.
Over time, these begin to spread and grow larger. The specks appear as reddish brown pits which, unfortunately, dont reproduce well in the photos weve provided.
Steel Wool and Sea Water
To create a more photogenic test, we doused the steel wool with sea water, duplicating our test of last summer. Again we used the same five brands and kinds of oils as with the cams and we soaked our steel wool pads with each oil for several hours. We dosed one pad with Corrosion-X and left one bare, sans treatment of any kind.
We added the Corrosion-X trial as an interesting baseline, at the suggestion of oil chemist Ed Kollin, who helped design the formula. We were curious to see if it could absolutely, positively stop corrosion in such a harsh and unrealistic test. We were assured that it could.
We hung each pad as shown in the photos and let them drip for a day. We then applied an equal amount of sea water via a spray bottle for three days. Within hours, the un-treated pad started to drip rusty water.
Upon inspection overnight, the pads treated with Shell W100 and Phillips XC also showed signs of rust while the pads treated with Shell W100 Plus, AeroShell and Elite were relatively free of rust. Within 24 hours, the Shell pads were dripping rusty water and showing signs of rust noticeably more than the Elite pad. The Corrosion-X pad looked nearly brand new, with not so much as a speck of brown or red. The untreated pad was fast turning into a crumbling blob of powdery orange and brown corrosion.
By the following day, we led the mechanics in the shop through a blind selection process, asking them to pick the least corroded sample. The results were unanimous: Of the oil soaked pads, Elite showed slightly less rust than the Shell products with anti-corrosive additives, namely AeroShell and W100 Plus. Calling the difference dramatic may be overstating the case but calling it noticeable certainly isnt.
The oils without any anti-corrosive additives- Phillips XC and Shell W100-had noticeably more rust, perhaps twice as much as the Shell samples with anti-corrosion additives. Interestingly, the Corrosion-X pad was unscathed, looking just as it came out of the box.
Why Stuff Rusts
Why engines rust and how oils prevent it is somewhat of a dark science. In our efforts to learn more about the performance of oils, weve always been somewhat shocked to learn how little research the engine manufacturers have done on this topic.
The theory is easy to grasp. One of the byproducts of internal combustion is water, most of which trails out the exhaust pipe or engine breather. But for every pound of fuel burned, the engine needs 10 or more pounds of air, which adds yet more moisture to the equation.
Some of that water gets past the piston rings and into the oil, where its entrained but condenses when the engine cools. Released water finds its way onto cams, cylinders and other shiny surfaces where it will immediately begin to cause corrosion, unless something-the oil-is there to prevent it. The fact that entrained water may be slightly acidic from dissolved combustion gasses encourages corrosion all the more.
Over the years, on both the automotive and aircraft fronts, oil manufacturers have experimented with additive packages to both reduce wear and prevent corrosion, although auto oils tend to have more sophisticated additives than aircraft oils. In fact, aviation oils are said to be relatively crude by comparison.
Why? Simple. Economics and liability are the prime factors. In the grand scheme, GA oils represent a tiny fraction of the businesses of Shell, Phillips and ExxonMobil but they account for a significant portion of potential product liability.
Memories of Mobils AV1 are still fresh in the minds of many aircraft owners and while the company doesnt officially consider that product a failure, it was certainly a public relations disaster for Mobil.
Nonetheless, if it had a chilling effect on future GA oil development, ExxonMobil didnt seem to notice, rolling out Elite aggressively last spring.
In its Elite product, ExxonMobil has introduced what it claims is the most aggressive anti-wear and anti-corrosion additive package available to aircraft owners.
For years, Shell has had the lions share of the GA oil market, Exxon, Phillips and others sharing the rest. We were curious about additive packages in AeroShell and Phillips XC and were somewhat surprised to learn that of the two, only AeroShell has specific anti-corrosion additives. And according to Ben Visser, who recently retired as Shells aviation oil program manager, AeroShell got its anti-corrosion additive package only about eight years ago.
While Shells straight-weight W100 still has no anti-corrosion additive, the recently introduced W100 Plus does. Visser told us the new straight-weight was introduced primarily to meet demands for what customers perceive as superior corrosion protection by straight-weight oils over multi-weights.
Yet Visser says Shell has always maintained that theres no measurable difference in corrosion protection between multi-weight and straight-weight oil. Phillips has the same view. The counter view stems from engine shops which insist that some engine run on multi-weight are more susceptible to corrosion than those run on straight-weight oils.
What does all this mean? Is Elite the clear winner of the anti-corrosion war? Although were not yet willing to crown Elite the new oil king, our informal tests are certainly convincing if not conclusive. Once again, Elite seems to be measuring up to ExxonMobils claims.
As an aside, were becoming increasingly skeptical of anecdotal claims that straight-weight oil is better at preventing engine corrosion than multi-weight is. We simply dont see any evidence of that in these tests, an observation that supports oil company claims that its the additive package that matters, not whether the oil is straight-weight or multi-viscosity.
In our view, anti-corrosion oil additives make a marked difference and are not something to be discounted out of hand. If youre a typical owner-and that means you dont fly your airplane every week-we think our most recent round of testing strongly favors Elite strictly on the anti-corrosion issue.
ExxonMobil has made impressive claims that Elite has superior wear prevention characteristics as well and on that count, the jury is still out. It hasnt been in the field long enough for us to make an intelligent judgment. In a future issue, well be examining wear properties through lab tests were commissioning.
In the meantime, if corrosion is a worry for you because you dont fly much, you live in a humid area and/or you have an engine with nitrided cylinder barrels, we think Elite is the best choice for anti-corrosion properties, followed by AeroShell 15W-50 multi-weight and Shell W100 Plus single-weight oil. Unless you run your engine regularly up to temperature every few days, theres little doubt that internal corrosion is a concern. Once started, corrosion may accelerate wear at an alarming rate and cause a needless early overhaul. If a decent oil can prevent it, why not give it a try?
-By Coy Jacob
Coy Jacob is a long-time Aviation Consumer contributor and head of the Mod Squad in Venice, Florida, formerly the Mooney Mart.