by Paul Bertorelli
Call us cynical, but we always enjoy a bitter guffaw when we see the phrase aircraft quality attached to ad copy aimed at people who dont know much about airplanes. For those who do, aircraft quality means the part costs four times as much as it should, is about as sophisticated as a bag of gravel but only a Yugo can be counted on to break less frequently.
Nonetheless, were making progress, even in the mundane world of aircraft engine oils. Until quite recently, aircraft oils dwelled in what might be generously considered a technological backwater. The base and additive packages traditionally havent been as chemically sophisticated as those found in the typical automotive oil, where liquid cooling and the use of unleaded fuels opens up a range of additive options not usable in air-cooled aircraft engines.
And compared to the passenger car motor oil market-PCMO in industry speak-the aircraft engine oil market is a cottage industry, so its understandable that the big research dollars are spent elsewhere. The car industry has something else the aviation market lacks: comparative long-term testing of oils by independent trade groups and research councils. Even the engine manufacturers and engine overhaul shops make oil recommendations more rooted in history and anecdote than in hard data.
This means that youre on your own to sort through the ad hype to decide what kind of oil to use in your expensive new overhaul. Advertising claims and anecdotal reports from engine shops cloud the question of which oil is best for which engine. In our view, an owner should be concerned about two things in buying oil: anti-wear characteristics and the ability to inhibit corrosion, a significant cause of premature engine damage.
For this article, were again examining the corrosion-protection performance of popular aircraft piston engine oils. Specifically, for this round of trials, we wanted to address the question of whether straight-weight oils are better at preventing rust in engines than are multi-weights-they arent, to give you a sneak peek-and whether a new oil additive called ASL CamGuard lives up to its claims. Among other properties, CamGuard is supposed to improve corrosion inhibiting. (Another peek; it does.)
Worth noting is that we didnt test all of the available oils, either multi-weight or straight-weight, for the simple reason that they arent available everywhere. Chevron, for instance, has a line of piston-engine oils thats all but impossible to obtain in many parts of the country. BP/Castrol has recently announced a new line of straight-weight aviation oils and although we tried mightily to obtain samples, the case sent to us a month later arrived too late for our testing project. Well follow up in a future issue.
How We Tested
Our test method is summarized on the facing page. We offer this caveat: the inside of a crankcase is not easy to duplicate, thus various lab solutions have been devised to test corrosion protection. Exposure of steel parts inside a humidity cabinet is one means of testing but critics of this method say it lacks real-world credibility because temperature and humidity are held constant, which clearly isnt the case in the real world. Inside a parked aircraft engine, humidity and temperature vary with the season and time of day, although one hopes the innards wont be exposed to salt spray. An engines surfaces are exposed to moisture, however, and to weak acids that are byproducts of combustion.
In our previous corrosion testing of oils and rust inhibitors-we have used both the salt spray method and another protocol involving dilute hydrochloric acid in a sealed container exposed to diurnal temperature change. Exxon complained that hydrochloric acid isnt a byproduct of engine combustion, but sulphuric acid is, so for this round of testing, we used both salt spray and acid to accelerate the corrosion process in two separate rounds of trials.
For the salt test, the steel was treated with oil and exposed to the open air. The metal was periodically sprayed with a 20 percent saline solution.For the simulated crankcase test, we used a sealed plastic container with a liter of 1 percent solution of sulphuric acid above which the samples were suspended. The container was alternately heated to about 90 degrees and cooled to simulate diurnal conditions.
While our trials arent the equivalent of laboratory corrosion testing, we think they are a fair and representative way to see how oils can be expected to perform in real world conditions. Exxon is, as far as we know, the only company to release at least sketchy results of comparative testing, claiming its Elite is better than any other oil at preventing corrosion. If thats true, our trials ought to confirm that. (With some qualifications, they did.)
For its corrosion tests, Exxon used a test called ASTM D 1748, which involves exposing parts in a humidity cabinet at 100 percent humidity and 120 degrees F. For its metal samples, Exxon used actual engine parts, specifically new valve lifters. The public-consumption version of its report includes photographs of the corroded lifter surfaces, showing that the surface treated with Exxon Elite has less rust than one treated with a commercial 15W-50 oil which we take to be AeroShell, even though the report doesnt say as much. (Exxons published wear tests do name names, specifically AeroShell and Phillips XC.)
Our open-air salt spray test is a far harsher challenge to an oils corrosion prevention characteristics than is a humidity cabinet, but its equally harsh on all the oils so we think its fair. We noted that rust was much slower to form in the sealed container with the acid solution but that the oils performed similarly in staving off rust.
The graphics (see sidebar) illustrate both the test method and the results. For our last round of testing, we tallied up the days each oil protected a metal sample to 50 percent corrosion coverage. This time, we pegged the test period at 14 days, then carefully measured the corrosion coverage on each metal coupon as explained in the graphic. Although some degree of subject judgment is required, the grid method we used is as unbiased a means of judging coverage as we are able to devise.
Once again, our tests showed that Exxon Elite and AeroShell 15W50 were the top performers in preventing corrosion. Our experiments show that Elite enjoyed a slight margin over AeroShell, which tends to support Exxons claim that its tests showed superior corrosion protection over other oils. However, the advantage is slight enough that we consider the two essentially equal. In our previous round of testing, we found that AeroShell protected the sample against 50 percent corrosion coverage for a day longer than Elite did. In our view, thats not enough of a difference to declare one better than the other.
Again, Phillips XC 20W50 finished well behind Elite and AeroShell, as well it should, for Phillips makes no claims for this oils rust-prevention qualities. Examining the metal samples side by side without benefit of the grid, the XC-treated metal was clearly more corroded and, in spots, had deeper pitting than we found on the Exxon and AeroShell-treated metal. One surprise contender was a Phillips 20W50 multi-weight mineral oil called Type M, which performed better than XC and better than all the others except Elite and AeroShell. One reason for that appears to be that it lacks an ashless dispersant additive, which the other oils have. This may improve the oil films water exclusion capability.
We dont have the space to graph the test results using the weak sulphuric acid solution but the oils performed in essentially the same order: Elite had 2 percent corrosion, AeroShell 15W50 had 3 percent, Phillips XC was 8 percent corroded in the AeroShell W100 straight-weight showed 6 percent corrosion. Obviously, whether the acid test better duplicates a crankcase or not, its a less harsh test than the salt mist.
The claim that straight-weight oils are better at preventing corrosion than multi-weights follows this oft-cited logic: because theyre more viscous than multi-weights, they cling to the metal longer and offer better protection.The anecdotal field confirmation of this is the so-called drain-down test.After shutdown, the engine filled with multi-weight will show a higher oil level than one filled with straight-weight, thus proving the contention that more of the oil remains on critical parts, thus inhibiting rust formation.The straight-weight filled engine will continue to make oil long after the multi-weight has drained down.
The oil companies-at least those marketing multi-weights-have dismissed this claim and our trials support that view. Right from the bottle, straight-weights were noticeably poorer at protecting the metal against the ravages of corrosion. Only one straight-weight oil we tried, Shells W100Plus, has an anti-corrosion additive and it showed, albeit not much. W100Plus performed about equally with Phillips XC, allowing about 81 percent of corrosion coverage during the two-week test period.
Two straight-weight oils without additives-Aeroshell 100 and 120-did poorly in our trials. In fact, by the end of the two-week test period, these two products were as badly corroded as the unprotected control sample.Lacking any additives, Shell makes no claim for corrosion protection for these oils. Our view is that if youre going to use a straight-weight, stick with one that has additives, such as W100Plus, or add your own, such as CamGuard.
Pro and con arguments over straight-weight and multi-weights have risen to Biblical intensity and with no convincing independent test data, these yield more heat than light. Critics of multi-weights argue that the additive packages that improve their performance when fresh degrade, leaving behind a thinner oil thats less able to cling to and protect parts against corrosion and wear than a heavier straight-weight would.
Another layer of the argument is that some multi-weights are blended semi-synthetic products-specifically Exxon Elite and AeroShell 15W50-and synthetics are known to be less effective than mineral oils at suspending lead combustion byproducts. Straight mineral oils have better solvent properties than synthetics so theoretically, a semi-synthetic could allow more lead-based sludge to gum up oil passages while its anti-corrosion properties degrade with time in service.
Could this account for the anecdotal reports of less corrosion with straight-weight, mineral-based oils? We shopped the idea with the oil companies.
Predictably, theyve heard these claims and just as predictably have explanations to blunt them. We know of no link between lead and rust formation, says Exxons Stephen Sunseri, who oversees the Elite product line. The dispersants job is to chemically attach itself to the impurities in the oil, including lead, and keep them uniformly suspended so that you can drain them out with the oil. The more effective your dispersant, the better your sludge and deposit control.
Although its accepted that mineral oils add solvent qualities to this equation, theres no convincing data that semi-synthetics leave behind enough lead byproducts to plug engine oil passages, the most compelling worry about sludge.
What about the notion that additives, both the viscosity modifiers in multi-weights and the anti-corrosion compounds, wear out with use? The fact is, they do, but not enough to matter, according to those in the industry.The elastomers used as viscosity modifiers in multi-weights work at the molecular level by binding together and thickening the oil in response to higher temperatures, reversing the process when cool.
Mechanical meshing of engine parts-so called shearing-degrades the viscosity modifiers performance by literally chopping up the molecules. The companies say this degradation is minimal. Modern multi-weights are considered so highly shear-stable that any degradation in use is trivial. Under extreme pressure, theres some shear loss at the top end, says Phillips Harold Tucker, but it would never shear out a whole grade.Ed Kollin, an independent oil chemist who developed the CamGuard additive, agrees. He thinks AeroShells viscosity modifiers are the most shear-stable but those used by other companies vary only by percentage points, if that.
An owner using a straight-weight doesnt have to worry about this and straight-weight oil, variable by brand, is about a third cheaper than multi-weight oil. For that price difference, the multi-weight user gains the convenience of not having to bother with seasonal oil changes and only one type of oil need be carried in the airplane for make-up. A multi-weight gives better start-up lubrication across a wider range of temperatures by dint of its lower viscosity in cooler weather.
Shells Paul Royko told us that while our bench tests are interesting, the real measure of an oils performance is in the crankcase. Bench tests are useful, but you can tweak them to see the flavor youd like to see, he says.We cant argue the point and when we asked Royko for Shells data on in-crankcase performance, he promised to provide us with this for a future article.
In the meantime, Shell-which has a line of mineral straight weights-adopts the view that selecting a straight-weight over a multi-weight isnt as simple as one being good and one being better. There are climate, engine and operational considerations that complicate the choice. (More on that later, too.)
Some shops who recommend straight-weight oils argue that the anti-corrosion packages in all oils-including multi-weights-wear out with use. CamGuards Kollin says theres truth to the claim but probably not enough to matter for owners who change oil at sensible intervals, say 25 to 40 hours. He explains that rust inhibitors work by attaching themselves to metal surfaces to provide a molecular barrier against water and acid intrusion. As parts are exposed to metal-to-metal contact, the barrier is worn away but the oil should have sufficient rust inhibitor in solution to replenish it. Fresh oil presumably does this better than old oil and Kollin says CamGuard improves the process.
Not that everyone agrees on rust inhibitor additives or even that aircraft oils should have them at all. Phillips Harold Tucker says XC has no rust inhibitor and the company makes no claims about anti-rust performance. XC does have an additive to neutralize corrosion-causing combustion byproducts.Tucker says Phillips hasnt ruled out adding a rust inhibitor but it wants one that will work well. I dont see much difference between a rust inhibitor that protects for two days and one that protects for five days, he says, adding that rust inhibition is a tradeoff.
I can add an inhibitor that will absolutely blow rust away, stop it cold, he says. But then I have to worry about bearing weight loss. Shell ecnountered this in 1998 when a change in formulation caused cop per leaching in engines. Although that was more of a nuisance than a real threat to engines, no one likes seeing metal spikes in oil analysis and Shell corrected its additive package.
Exxons Sunseri agrees about balancing additives and says thats one reason why additives like CamGuard arent necessary. The cost of supplemental additives is typically money you can save, he says, agreeing with Tucker that additives can work against each other.
Based on these trials, which oil is best and should you go with a straight-weight or a multi-weight? Ignoring the other important aspect of oil performance-anti-wear properties, which well examine in a future article-our conclusions about these oils for anti-corrosion purposes remain essentially unchanged.
For multi-weights, Exxons Elite and Shells 15W50 are nearly equal performers in preventing corrosion. We cant quibble with Exxons claim that Elite is better at rust prevention; our tests showed this. Were standing by for Shells data that says were all wet. Considered together, we think both Elite and AeroShell 15W50 distinguish themselves from the other products as superior corrosion fighters. As for the multi-weight versus straight-weight screaming match, the inarguable advantage of multi-weight is convenience.Its easier to use just one type of oil year around than it is to worry about seasonal switchovers.
As for performance, the theory that the multi-weights additives disappear with time in use simply isnt supported by any science or data that we can find. Further, our tests have repeatedly shown that fresh out of the bottle, straight-weights arent more effective than multi-weights at preventing corrosion. At best, theyre comparable.
Straight-weights without rust inhibitors are actually worse. Weve talked to as many engine shops about this as anyone and if just one shop-just one-would show us other than anecdotal evidence that multi-weights promote corrosion, well go on a holy crusade against them.
If you prefer a straight-weight oil-they are cheaper, after all-it makes no sense, in our view, to use one without additives, which argues for selecting a product like AeroShells Oil W100 Plus over its Oil 100, which is really considered a mineral break-in oil. We dont agree with the Phillips view that anti-rust additives dont make much difference. We would rather have some protection than none at all and if rust does get started, the less of it, the better. Inhibitors clearly help.
What about CamGuard? At $25 a pint at each oil change, is it worth it?Considering just its rust inhibitor properties, using it will add a buck an hour to your operating costs, if you change oil at 25-hour intervals. If you fly infrequently and youre worried about corrosion due to disuse, we think the data-both ours and CamGuards own detailed test results-suggest it has merit.
Before considering it a must-have, we would like to see more real-world field-test experiences. So far, however, we like what we see and will be conducting field trials with it in our own airplane. If you decide to try it, we recommend careful monitoring of wear metals through regular oil analysis.