Answers About Oil
We like to think we remain instantly abreast of every important development in general aviation and now we're in possession of a scientific discovery only we can reveal to the world: If you want your phone to ring off the hook, your fax machine to choke and your e-mail inbox to overflow, publish an article comparing piston engine oils for aircraft.
We did that very thing in the February 2005 issue of The Aviation Consumer and although the article was based on the most extensive oil testing we've done to date, it generated an enormous number of what we would call "yeah, but" questions. "Hey, yeah, great article on oils, but ..." wrote one reader, asking detailed and specific questions about his current favorite choices, Phillips XC and Exxon's Elite. Another reader phoned to ask which oil would be best for his twin Cessna, which is flown under 100 hours a year. Would AeroShell 15W50 be the best choice and should he add CamGuard to it?
The harsh reality? We simply don't know. The more we burrow into the subject of engine oil, the more we find more heat and smoke but less light. If there's any constant truth in the aviation piston engine oil field, it seems to be that there are too many variables to make one-size-fits-all decisions about which brand or type to use. An engine that's flown daily or several times a week will probably sail right to TBO using any oil. The hard case is the occasionally flown engine hangared or tied down in a corrosive climate; it will need all the rust protection it can get and even the best oil might not save it from an early demise. Unfortunately, this scenario is closer to the typical GA engine than not.
So much for our informed opinions. For this article, we've contacted the major oil companies -- Exxon, Shell and Phillips -- and presented them with the kinds of questions readers have been sending us. These extensive interviews haven't produced take-it-to-the-bank advice on any single engine oil but we think that the observations of the people who make these oils have merit and should be considered when deciding which oil to use.
The aircraft piston engine oil market appears to be dominated by Shell with its AeroShell 15W50, along with various mono- or straight-grade products. Phillips and Exxon share the remainder of the market, although not necessarily equally, along with lesser players such as Chevron and BP Castrol. The latter two brands are so difficult to obtain in most parts of the country that we're not sure they're realistic choices.
For this article, we spoke to Stephen Sunseri and Shlomo Antika at Exxon, Larry Norris at Phillips -- now PhillipsConoco -- and Robert Midgley, Shell's global technical director. We posed a series of questions from readers to each technical representative. Answers from Sunseri and Antika are combined in a single response.
Multi-Grade vs. Mono-Grade
Q: In our past research on engine oils, we get the impression that oil companies believe multi-grade products are better than mono-grade oils. Is that a fair assessment? If so, why do you still have mono-grades in the product line?
Shell/Midgley: We sell more of Aeroshell W100 straight-weight oil than anything else. It's purely legacy. Most things in this market are fairly slow moving. You get a lot of inertia. Our philosophy is to give people choices, educate them as best you can and let them decide. AeroShell 15W50 multi-grade has the most complete additive package but it's also the most expensive. In having straight-weight oils without the additives, we are giving a choice to the buyer who doesn't want or need the additives and who doesn't want to pay for them.
Exxon: This is one of the areas we get the most questions about. We do have mono-grades -- the Exxon Aviation Oil EE line -- and have had them for 60 years. About seven years ago, we brought out a multi-grade that we believe outperforms the mono-grade products; we believe multi-grades bring a lot of advantages to the table. But the reason we retain mono-grades in the product line is for the simple reason that a sizeable proportion of our market still prefers a mono-grade oils. For our market, we still sell more mono-grades than the Elite multi-grade. But the lines are going to cross shortly and Elite will be the larger seller.
Phillips/Norris: There are people out there who have been taught they should use single-grade oils and they've gotten good results with them. There's a market there, so we continue to offer them. Based on the fact that we get a lot of questions on this subject, the level of understanding about multi-grade oils is not good. Multi-grades have been in this business long enough to have become the premium oil of choice. But we'll continue to offer mono-grades until it doesn't make sense to support them any longer.
Q: We've been told by many owners that they believe mono-grades have a superior ability to prevent corrosion. Even though our tests prove otherwise, this still makes sense to us, because the mono-grades appear thicker and take much longer to drip off parts back into the sump. What's the technical view?
Shell/Midgley: I give a certain level of credence to that, perhaps for the short term. But over the longer term, our testing shows that corrosion protection of mono-grade versus multi-grades tends to even out over time. In other words, long term, the differences aren't great. Corrosion performance can be as much as function of operation as oil formulation. Testing seems to favor the multi-weights over short periods of time, but over longer periods, the differences are less noticeable
Exxon: Testing simply doesn't support that notion. We've done lots of it and we just don't see the advantage of mono-grades for corrosion control. There is some improved corrosion protection in mono-grades with the Lycoming-specified additive but you're still not getting the robust protection that we have in our multi-grade oils, specifically Elite.
Phillips/Norris: For some reason, people think multi-grades are thinner than mono-grades but they really aren't. In the engine, in the ring area, where the oil really has to work hard, the multi-grade will maintain viscosity better than a mono-grade will. It's better at heat dissipation and at sealing the rings. As for taking longer to drip off parts, I've never concluded anything like that. As the mono-grade cools, it will become more viscous, but we're not talking orders of magnitude.
Q: Given what remains a strong, but not growing, preference for mono-grades, why not put the sophisticated additive packages in those oils?
Shell/Midgley: In the main, cost. If you're running a flying school and you want to make money, you don't want to pay for an additive package you really don't need. For our customers who want a straight-weight oil with additives, we have AeroShell Oil W100 plus, which contains the same additive package we use in AeroShell 15W50. If an engine is flown frequently, it doesn't necessarily need an anti-corrosion additive because the engine parts remain coated with oil. The bearings and cam surfaces are fully protected so it probably doesn't need an anti-wear additive to protect dry surfaces at start-up after the engine hasn't been operated in some time.
Exxon: We have not considered that. Some specialty applications -- radial engines, for example and some military applications -- need mono-grades. But for the largest portion of our market, we think multi-grades offer the most advantages and that's why we've decided to put the most sophisticated additive packages in Elite.
Phillips/Norris: We consider the details of the additive packages to be proprietary. Only one of our mono-grades, the Type A 100AW, has a stated additive, which is the Lycoming additive.
Q: Compared to the automotive sector, aftermarket additives for aircraft engine oils are few. One, AvBlend, has been around for awhile and another newer one, CamGuard, is drawing interest. What's your view and recommendation on these?
Shell/Midgley: Use them if they are fully tested. But generally, they aren't tested to the same degree our base oils are. There can be interactions between additives and the base oil chemistry. For example, if you improve corrosion resistance with an additive, do you prevent the anti-wear additive from working or do you make the oil's load carrying capability worse? You don't know. FAA approval is on a "no-harm" basis, but that doesn't mean the additive improves the performance of the oil.
Exxon: We don't know if these additives diminish the performance of our oil or not. We haven't heard any negative comments about them and we don't have any test data. As you know, our view is that buying these additives is a cost you can avoid as long as you're using a quality oil.
Phillips/Norris: We would not sanction their use, other than the Lycoming additive, mostly because we don't have information on their performance and effect on the oil. There's a lot of testing we go through to certify an oil. Beyond that, once you have a formula, there are engines to run and hours to generate to prove the oil. We just don't have that for the additives.
Q: Shell and Exxon use semi-synthetic basestock for their aviation oils while Phillips uses a mineral basestock. Given the problems Mobil encountered with its AV-1 synthetic aviation oil, why do you feel synthetics are an advantage or a disadvantage. Will we see another all-synthetic aviation oil?
Shell/Midgley: All synthetic? Not as long as we have lead in the fuel. Fully synthetic oils have no lead-carrying capability. We know of no additives that will address this because lead byproducts in an engine aren't a single species; combustion forms many lead compounds in the oil. Designing a single additive to address this would be difficult. Synthetics have definite advantages, especially better viscosity characteristics generally. That means you have to add less in the way of viscosity index improvers, which is good.
They potentially extend drain periods so for a commercial operator, he may spend more money on the oil but save money in the long term because he changes it less often. But for the average owner/pilot who flies less frequently, the benefit may not be there in a pure synthetic.
Exxon: That's an interesting question. The answer has a lot to do with how long we'll be using leaded fuel in aircraft engines. The fact is, it (a pure synthetic aviation oil) can be done. The question is whether or not it's worth it. What would the reception in the market be versus what it costs to develop and make? The cost could be astronomical and we're not sure owners would pay it. We think people are willing to pay a little premium for a better oil, but there's a limit.
We chose a blended synthetic basestock for Elite for a number of reasons. A synthetic has an inherently better response to temperature change; they change, but less rapidly than mineral oils. That allows us to use less in the way of viscosity improvers and we think multi-grade synthetics are more resistant to shearing. We're aware, of course, that Mobil had problems with Mobil AV-1 related to lead solubility. At the time we were developing Elite, we didn't have benefit of what Mobil learned about all-synthetic oils for aircraft engines. By the time Exxon and Mobil merged, Exxon Elite was essentially commercial. But we still would have picked a synthetic blend because we think it has clear advantages. We agree from our automotive experience that synthetics bring good things to the table; stronger film strength, better viscometrics and oxidative properties.
Phillips/Norris: Why not a synthetic for Phillips? It's mostly an issue that we've got a long history of success with XC which, as you know, uses a mineral basestock. We've had a lot of loyal users with excellent success using it. I'm not sure I see the advantage of bringing in synthetic basestocks at this point. Synthetics can give you extended drain time and better low temperature performance but in the aviation environment, I'm not sure we see these as worthwhile technical advantages. Phillips has experience in synthetics in the automotive market but we haven't seen good reasons to bring that into aviation.
Q: As we've reported previously, corrosion control seems to be a major worry for owners. Was your oil formulated with that as the leading goal and what are you doing to improve corrosion resistance?
Shell/Midgley: You need to balance all these things. Corrosion protection is undoubtedly an issue but it's not the only issue. Cold circulation at start-up is a concern; you want to formulate an oil that provides some degree of protection between parts until the oil gets there after start-up. If those parts encounter a higher load than intended, you can get fatigue wear. Engineers [mechanics] do a terrific job but what they see as parts failed due to corrosion have actually failed due to fatigue from wear at start-up. (See spalled lifters in photo at right.) Protection against oxidation is also important. How does the oil perform over time versus how it performs as virgin oil?
Exxon: Rust control rates among the top few considerations in Elite's design; any engine shop will tell you corrosion is a worry. We didn't prioritize these, but wear prevention and dispersant qualities -- when you drain the oil, does all the bad stuff go out with it? -- were also important. Although we're always looking for ways to improve the additive package, we don't have any current plans to change it. We're satisfied with the performance.
Phillips/Norris: [Norris declined to comment on the details of the additive package in Phillips XC. We have previously reported that XC has no rust inhibitors, although it does have an additive to prevent the oil from becoming too acidic and causing corrosion.]
Q: For years, we've heard the rumor that if they wanted to, the oil companies could develop an oil that would guarantee TBO. Is this so-called "super oil" economically or technically realistic? Further, are aviation oils simply not as sophisticated as car oils due to market size?
Shell/Midgley: If a super oil were possible, I'd make it tomorrow. I really try not to use terms like "technologically unsophisticated" when describing aircraft oils. At this time, there are no oils that will perform better than what we are producing now. In some ways, these oils are at the peak of their potential performance. We are limited by the fuels used -- leaded avgas -- and by engine design.
Large displacement, low-RPM air-cooled engines burn oil, which means we can't use more desirable additives because they will form deposits, potentially causing pre-ignition. When the industry produces water-cooled engines along the automotive line, we will be able to move forward with engine oil technology.
Exxon: We think that's a fair assessment. We have to look at product economics. We can spread development costs of automotive oils among the 260 million cars out there but there are only 200,000 airplanes. As for a super oil, there are very real limitations on additive usage for aircraft engines. Automotive oils use anti-wear metallic additives -- zinc and molybdenum, for example -- that are ash forming, so we can't use them in aircraft oils. Things may change if we begin using unleaded fuel but that still doesn't mean we can use ash-forming anti-wear additives.
Phillips/Norris: Frankly, the technology on the automotive side is moving at warp speed. It dictates a complete reformulation every five years. There have been significant changes in basestock technology, not just additives. We haven't seen that in aviation.
In automotive, the primary drivers are fuel economy and emissions, so we're seeing oils as light as 5W20 for cars. That puts demand on cooling systems and in cars, we have that control. For the most part, we aren't seeing and probably won't see crossover from automotive technology into aircraft. There's nothing pushing us in that direction now.