Does the oil you dump into your engine really matter that much? Some owners swear Aeroshell is the best oil going while others wont touch the stuff, preferring Phillips well-regarded XC.
Lacking any valid side-by-side wear data across a range of engine types, no one can really say which is best, although some engine shops are willing to try. Our view is that the way an engine is operated probably has more to do with engine longevity than the type of oil used.
And like most owners, weve tended to assume that the aircraft engine oil market is far too small to justify serious research into improved products.
We assumed wrong. Last spring, oil giant ExxonMobil announced a new piston-engine aircraft oil called Aviation Oil Elite and at this juncture, it looks as though theyll be putting the full weight of the Tiger behind marketing this new product.
Its too soon to judge if this oil represents an advancement in the state of the lubricating art, but at this juncture, were intrigued by ExxonMobils claims and looking forward to how this oil will perform in the field. Although Elite is not yet widely available, weve acquired a couple of cases and were using in a Mooney with a Continental TSIO-360.
With the exception of a few industry insiders, ExxonMobils announcement in April caught many by surprise, us included. For a company the size of this behemoth, we had always assumed they spill more lubricating oil than the entire world market uses for aircraft piston engines in a typical year. (No Exxon Valdez jokes, please.)
Evidently, thats not quite right. The market is somewhat larger and more profitable than we imagined, although with three major players in the field, were not sure if it will remain that way for long.
In aircraft engines, oil is a big deal; less so in cars. Since the 1950s-when overhauling a car engine was a common practice-the auto industry has vastly improved the durability of its engines. Whens the last time you overhauled or replaced a car engine?
Improved oils have played a hand in this. If you have some gray under your cap, you can recall that the recommended oil change interval for cars used to be 1000 miles. Its now three times that or more. In aircraft engines, the interval hasnt changed a bit, nor is it likely to, given the duty cycle of an airplane engine.
But oil is thought to play a much larger role in the durability of an aircraft engine than it does a car engine, especially with regard to holding up to high and spikey operating temperatures and protecting an engines internals against corrosion during periods of idle use.
That said, one major difference between automotive engine oils and aviation piston engine oils is that the much narrower framework for SAE/Mil Spec approvals which govern aircraft oils simply doesnt exist in the automotive world. While automotive oils must meet SAE specs, in practice, oil companies can practically do anything they want to meet these requirements and even exceed them for auto engine oils. Moreover, auto oils can (and do) vary widely from batch to batch and even from one bulk processing plant to another. Harold Tucker, Phillips technical director, told us that while GA oils are produced to tighter specifications for quality control, theyre quite simple chemically when compared to automotive blends, a point echoed by Elite project chief, Shlomo Antika at ExxonMobil.
In short, the oil you dump into your aircraft engine is largely pure base stock with only a few additives while your car engine oil has a sophisticated and ever-changing additive package.
A Mine Field
Not that the oil companies havent tried to produce better aviation oils for piston airplanes. But when they have, the results havent always been encouraging. It has been quite a while since a major oil company has introduced a major improved aviation oil with as much fanfare as ExxonMobil is lavishing on Elite.
Shells 15/50 was introduced in the early 1980s and Phillips XC in 1979. While Exxon has been marketing GA oils in the past, they enjoyed a relatively small market share and reportedly were leasing one of their competitors formulas and selling it branded as Exxon, further adding to the notion that this market is just too small for a big company to bother with.
ExxonMobils Michael Schaefer informs us that for some five years-and prior to the merger with Mobil-Exxon has been quietly working to develop what they claim will be a state-of-the-art blended semi-synthetic GA oil.
Not that Mobil had much luck in the aviation oil business. Mobils AV-1 synthetic product was thought to be revolutionary when it appeared in the mid-1990s in the wake of the companys very successful synthetic auto engine oil.
But because of poor lead scavenging properties, AV-1 turned out to be an unmitigated disaster that embroiled Mobil in lawsuits and eventually resulted in the company reimbursing some owners for engine damage related to the oils shortcomings.
Phillips had its night in the barrel with XC II, a supposed improved formulation that turned out to be anything but, producing problems with cam spalling and unusual wear.
One seemingly unavoidable problem is that engine manufacturers are nearly always the target for criticism when components fail or their engines dont make TBO. Some of the blame is inevitably shifted to the oil, often without convincing data one way or the other. Oil companies say the same only in reverse. Thats hardly fertile ground into which a new engine oil will take root.
Another factor is the ashless dispersant restriction in the current Mil Spec or, now, SAE J1899 Specs which prohibit GA oils to have any ash content, a compound which is common in most all other piston engine oils.
Ash content of oil is a by-product of detergent additives when combustion takes place. In the old days, ash residue was thought to build up in combustion chambers and possibly cause pre-ignition.
In aircraft engines, serious prolonged pre-ignition could cause nearly instantaneous upper end failures or, at the least, premature piston and valve wear.
All of the sources weve interviewed in the major oil companies admit they could produce or already have in-house improved GA blends, but wont bring them to market because of economics or potential product liability trouble down the line.
Phillips Tucker points out that he thinks its time the GAs prime Mil Spec/SAE guidelines to be changed to allow the addition of the newer detergent or ash producing compounds and other high-tech additives which have proven effective in automotive oils.
He echoes others in the industry who have admitted privately that they experiment with these improved super oils with their own company aircraft but wont bring them to market just yet.
Exxon Plunges In
But ExxonMobil may be about to change that with its new Elite product, which is what the company calls a third generation blend designed specifically for the way owners operate their aircraft in the real world.
While still limited to the constraints of Mil Spec/SAE J1899, Exxon appears to be advancing the cutting edge of oil by a measurable degree. Even Phillips Tucker says Exxon has upped the ante in the GA oil business.
Being the industry giant that it is, when ExxonMobil pays this much attention to a product line, everybody else in the business had better follow suite.
Exxons Elite is a semi-synthetic blend with a sophisticated additive package superficially designed for aircraft engines and not so tightly constrained by Mil Spec considerations that simply dont work for aircraft piston engines.
Elite is 88 percent synthetic petroleum basestock and 12 percent additives, including anti-corrosion and anti-wear compounds. Like Aeroshell, Elite is a 20-50W multi-weight intended for year round use in virtually all climates.
ExxonMobil has specifically formulated Elite to perform well in engines that arent flown much, hence it has what are claimed to be excellent anti-corrosion properties.
According to ExxonMobil, Elite oil passed a test called ASTM D1748, which exposes an uncoated steel panel to 100 percent humidity in a humidity cabinet until they show signs of rust. Claims ExxonMobil, Elite proteced the specimens from corrosion at least four times longer than conventional aviation oils.
ExxonMobil also claims much better viscosity control for Elite and has test data supporting the claim that its anti-scuff properties are better than other oils on the market, especially with regard to cam and cam follower spalling and pitting.
In one test severity test, ExxonMobil used cam followers from a Lycoming TIO-540-J2BD, a notoriously wear-prone engine, and jacked the valve spring load to 50 percent overload.
Repeating the test several times, ExxonMobil claims that the cam lobes and followers were severely damaged after only 180 hours, using an unnamed commercial 20W50 oil which we take to be XC. A 15W50 oil-sounds like Aeroshell-reached 300 hours before the parts showed severe damage.
When the parts were lubricated with Elite, on the other hand, no excessive wear was noted. Furthermore, Elite is supposed to protect engine internals against sludge and varnish build-up, a failing which sunk Mobil AV-1. ExxonMobil claims that independent experts examined engines run on Elite for 500 hours and concluded these engines were the cleanest they had seen.
Tall claims? You bet. ExxonMobil is essentially saying Elite will reduce wear, corrosion and result in a cleaner running engine. Some of these claims are based on lab data, some of actual tests in the field.
It may be two or three years before this oil has proven itself so we think any and all claims should be viewed with skepticism. Nonetheless, if these claims prove true, ExxonMobil may be on to something.
The Corrosion Issue
Engine longevity is a much sought after but seldom achieved goal of aircraft ownership. Whether its true or not, most pilots accept the conventional wisdom that a frequently flown engine will last much longer, easily making TBO.But how you fly it matters, too. Firing up the engine for a turn or two around the pattern may qualify as frequent use but it might be the worst thing you could possibly do. During normal operations, the engine oil collects moisture from the combustion process.
If the engine is flown for long enough-perhaps an hour or more-it reaches sufficient temperature to boil off the entrained moisture, thus reducing the amount that will condense on internal parts when the engine is shutdown. Theoretically, this reduces corrosion and even if it doesnt, it makes sense to have less trapped moisture in the engine.
Lycoming engines seem especially susceptible to internal corrosion on camshafts because the cam is located near the top of the engine. Warm, moist fumes tend to rise, condensing on the cams polished surfaces. Its generally accepted engine shop wisdom that many Lycs rust out before they wear out.
In fact, Ben Visser, Shells GA engine oil expert, points out that under certain adverse conditions, a typical Lycoming cam can show appreciable rust in as short a period of a few days using oil which doesnt have anti-corrosion additives. And as late as 1993, Visser notes that Aeroshell had no anti-corrosion additives, although it does now.
Corrosion performance-or lack of it-has sparked a vitriolic debate about whether to use multi-weight oils or straight weight oils. The straight-weight proponents insist straight weights cling to the parts longer and thus provide more protection, a claim the oil companies have always denied, complete with test data to refute such claims.
In fact, Shells Visser told us a few months ago that until very recently, when Shell introduced Aeroshell W100 Plus, Shells popular straight weight oils such as Aeroshell Oil W100 contained little or no anti-corrosion additives. To a degree, the oil companies recent emphasis on straight-weight oils has been because the marketing folks perceive that some owners want straight-weight oils. The guys who make the oil seem to uniformly prefer multi-weights.
We dont know what that does to the straight-weight versus multi-weight argument but lacking any evidence to the contrary, we still dont buy that straight weight oils offer better corrosion protection.
What To Do?
Given how confident ExxonMobil seems to be about Aviation Oil Elite, should you run out those few remaining bottles of Aeroshell and make the switch?
Well reserve judgment for now. Were impressed with what ExxonMobil has done and our own little backyard corrosion test seems to validate the companys claims, in a limited way.
The real acid test is long term usage. Since the track record on new oils is none too encouraging, were taking a wait-and-see view on recommending this new oil. Yes, weve switched to it for now and we arent especially worried about the consequences, given the test data weve reviewed. Well let you know what develops.
If youre considering switching over to Elite-and this advice applies to using or switching to any oil-keep several things in mind: Fly the airplane frequently and long enough to bring the oil to full operating temperature for at least an hour and change the oil at regular and short intervals. Short means about 25 hours if the airplane is flown infrequently, but no more than twice that if its flown lots.
If your engine isnt on oil analysis, it should be. Speculation about reduced wear is usually just that, speculation. But increased wear, especially on discreet engine parts such as valve guides, bearing and cams, will often show up in oil analysis, making sending off a sample at every oil change cheap insurance.
-by Coy Jacob
Coy Jacob operates The Mooney Mart in Venice, Florida. Hes a regular Aviation Consumer contributor.