What is it about oil that makes people crazy?
Historians looking back on the 20th century will probably point to oil, and its effects, as defining our era. So it is in aviation as in the rest of society. Owners, engine shops and manufacturers seem unable to agree on the best oil to use in aircraft engines.
Lately, the debate has focused on so-called straight-weight (often erroneously referred to simply as mineral) oils versus the newer multi-weight oils.
Ten years ago, everyone seemed to agree that the higher-priced multi-weights were better. Today, theres growing sentiment that cheaper straight-weight oils may indeed be the better choice, especially from a corrosion control point of view.
Oil serves five functions in an engine. First, it lubricates the internal parts by interspersing a liquid film between wear surfaces. This film cushions mechanical shock and prevents metal-to-metal sliding contact, which would lead to scuffing.
The oils base stock consists of either petroleum for mineral oils or synthetic petroleum substitutes for synthetic oil. Most oils contain a package of extreme pressure (EP) additives, which, due largely to concerns about pre-ignition from metallic additives, are tricrescyl phosphate and its successor, triphenyl phosphate.
Second, oil transfers heat from the hotter engine components to the sump and out the oil cooler. With temperatures in the ring groove areas approaching 500 degrees F, an important heat transfer factor is the oils oxidation stability. While synthetic and semi-synthetic oils have excellent oxidation stability, mineral oils need inhibitors to prevent heat-induced deposits.
Third, oil entrains, carries away and neutralizes contaminants in the engine. Detergents and dispersants are added to the base oil to keep combustion contaminants in suspension so that they can be removed via filtration. Additionally detergents serve to neutralize the acidic by-products of combustion.
Fourth, oil coats the internal metal parts of the engine and prevents corrosion-especially when the engine is idle. Finally, oil helps to provide a gas seal between the combustion chamber, especially the rings, and the rest of the engine.
Viscosity is the measurement of an oils ability to flow through a tube or orifice. In the lab, its measured via a device known as a Saybolt tube. The tube is immersed in a constant temperature water bath and the oil is allowed to flow through it. The time it takes for the oil to flow through the Saybolt tube is expressed in Saybolt Universal Seconds (SUS) or as its kinematic viscosity in mm2/s of flow. There are rough conversion formulas between the SUS value and the kinematic viscosity.
While talking about Saybolt Universal Seconds or kinematic viscosity might get you a date at the annual Association of Tribologists meeting, its unlikely to gather a crowd at the FBO. More common units for pilots are the SAE classifications (or weights) such as SAE 50 or SAE 20W-50. The standard SAE viscosity test involves running the oil, at 210 degrees F, through the Saybolt tube and converting the resulting SUS to the SAE numeric classification via a conversion table.
Theres another SAE viscosity test which involves running the oil at 0F through the Saybolt tube. When the test is run at 0F, instead of 210 degrees, a W is affixed after the SAE numeric classification.
Multi-weight SAE oils are tested for viscosity at both 210 degrees F and at 0F. Because of this, the viscosity for multi-weight oils is expressed with two numbers, such as 20W-50. The 20W indicates that the oil performs like a 20-weight oil at 0 degrees F and the 50 indicates that it performs as a 50-weight oil at 210 degrees F. This is important as at 0 degrees F, the 50-weight oil has a kinematic viscosity not two-and-a-half but 20 times that of the 20-weight oil.
An oils viscosity is one of its most important characteristics. Unfortunately, as with all of engineering, compromises have to be made. There is no single good viscosity for all conditions and purposes.
Factors favoring the use of a high viscosity oil include the minimum oil film thickness (MOFT) between engine parts. MOFT is highly correlated with oil viscosity. Get the oil viscosity too low and parts start banging into each other. A high-value of MOFT is critical for low RPM/high manifold pressure operation, which places additional stress on the piston rings, particularly at the tops and bottoms of the cylinder bores.
High MOFT (high viscosity) also helps limit secondary piston motion, such as rocking or slap. Both cause high levels of stress in the piston assembly and may, especially in the case of piston rock, deprive the ring area of necessary lubrication. On the other hand, low oil viscosity decreases the amount of energy lost to mechanical friction in the engine. Many pilots have noticed the difference in static RPM generated by a cold engine versus one thats been out flying. The difference is due to oil viscosity.
This fact hasnt been lost on automobile manufacturers. Theyve recommended lower and lower viscosity oils for cars not because cars need the lower viscosity oil but because it improves gas mileage. That earns them points with both the government and consumers.
Low viscosity oil also flows better, which helps it carry away more heat than a higher viscosity oil. More important, the better flow rate of low viscosity oil allows it to reach distant areas of the engine more rapidly, a critical quality during engine start.
The oil companies found that by adding long-chain polymers to the base oil stock, they could slow loss of viscosity that accompanied increased temperature. These so-called viscosity improvers (VIs) allowed lubricating oil to provide sufficient viscosity under high engine temperatures without also exhibiting excessive viscosity at low temperatures.
This was great news for cars. Oil could be thin enough to ensure easy starts and rapid distribution through the engine in even the coldest climate. At the same time, the viscosity improvers ensured that the oil would do its job at higher operating temperatures.
Multi-weight oils came into their own in the automotive world during the late 1960s and 1970s, eliminating the need to replace crankcase oil as the outside temperature changed. The auto manufacturers found that engines lasted longer as multi-weight oils allowed full lubrication at start up and freedom from excessive thinning at operating temperature.
In aviation, however, multi-weight oil was slow to catch on. But attitudes changed and owners and pilots started to believe that multi-vis oils could help their engines.
This belief was spurred on by the oil companies, who pointed out that as aircraft utilization began to decline in the 1980s, a multi-weight oil would be better for engines that saw infrequent use.
Two reasons were given: one the multi-weight oil would pump more rapidly throughout the engine, decreasing start up trauma for a motor standing idle for long periods. Second, multi-weight oil was pitched as an all-season oil. An owner need no longer worry that the oil appropriate for the end of August might not be appropriate for November.
Beginning in the early 1990s, attitudes about single and multi-weight oils shifted again. A number of operators were switching back to single-weight oils, especially during the summer months.
One reason given for the switch was the belief that single-weight oils provide better corrosion control than multi-weights, especially for infrequently flown aircraft. The argument is that at room temperature, single-weight oils were thicker than multi-weights and a single-weight was more likely to adhere to engine parts at shutdown.
This view is strongly supported by many engine shops which insist that the single biggest cause of pre-mature engine wear is corrosion caused by lack of use. A few shops weve spoken to argue that continuous use of pre-heaters during the winter aggravates the corrosion problem. As a result, some shops strongly recommend only single-weight oils.
That said, multi-weight oils have no inherent lubricity advantage over single-weight oils. In fact, some argue that the lubricating qualities of multi-weight oils were worse than those of single-weights because the multi-weight oils had to sacrifice some lubricant (i.e. base stock) to make room for the viscosity improvers. Multi-weight detractors point out that not only did one of the major oil manufacturers continue to produce a single-weight oil, but another manufacturer-Phillips-recently reintroduced a single-weight product. It seemed clear that the manufacturers, by their actions, were saying that they too werent wild about multi-weight oil.
The Company Line
We called on a couple of the largest aviation oil manufacturers, Phillips and Shell, to try and make some order out of the chaos. We found that while they agree on the main issues, there are differences in the details.
Shell and Phillips are adamant that both straight-weight oils and multi-weight oils are sold merely to satisfy different markets. Neither company says that straight-weight oil is superior in any way to multi-weights. However, both felt significant pressure to provide a single-weight oil to satisfy some consumers.
Shells Ben Visser and Phillips Shawn Ewing told us that their single-weight and multi-weights are identical in lubricity characteristics. In addition, Ewing pointed out that Phillips multi-weight oil stayed in grade-meaning it doesnt catastrophically lose viscosity-longer than single-weight oil, once the oil is warmer than 210 degrees F. This attribute is especially critical in the piston ring belt area.
Ewing and Visser also pooh-poohed the idea that multi-weights, because theyre thinner at room temperature and thus might strip off parts easily, offer poorer rust protection than straight-weights. Both point out that at shutdown, while the engine is still hot, both straight- and multi-weight oils have roughly the same viscosity and propensity to drip off of engine parts and into the sump.
Indeed, Visser and Ewing argue that their companys respective multi-weight oils were better at preventing corrosion within the engine than the straight-weight offerings. Visser made no bones about it: Our 15W-50 multi-weight controls corrosion better than any other oil that we or our competitors make.
Ewing provided a scientific explanation for the improved corrosion resistance of the multi-weight: He says the polymers used in Phillips viscosity improvers have a greater affinity than the base oil for the metal used in the engine. They remain on the metal longer, providing increased corrosion protection.
Resistance to the loss of viscosity due to mechanical shearing of the molecules which comprise the viscosity improvers in oil (VI) is termed shear strength. During operation, the VI molecules are literally torn apart by the various moving and sliding components in the engine.
Multi-weight oil detractors point to this degradation as another reason to stay with a straight-weight oil. Research appears to back that claim. One report, SAE930690, found that automotive multi-weight oils can lose a significant amount of their viscosity stability within just 500 miles of driving or 20 to 30 hours of operation. Visser counters that Shell simply doesnt see that effect in their semi-synthetic 15W-50 oil. He says tests show a permanent shear down of roughly 3 to 4 percent after 40 hours and he doubts its much different for the other manufacturers.
Phillips Ewing attributes this stability entirely to the VI package used in aviation oil. According to Ewing, aviation oil uses higher-quality (and more expensive) shear-stable VI improvers, whereas the VI improvers in automobile oil are non-shear-stable. Whats more, Ewing says, aviation oil dispenses with the more virulent detergents and other chemicals used in auto oils because those chemicals contribute to VI shear.
Visser is careful to point out that in addition to long-term permanent shear loss, temporary shear loss can occur under extreme pressure. He explains that synthetic and semi-synthetic oils (such as Shells 15W-50) have a leg up over conventional mineral-based oils (such as Phillips 20W-50) in high-stress, temporary shear conditions.
According to Visser, temporary shear loss in a 50-weight mineral oil can reduce its effective viscosity to that of a 5-weight. The same temporary shear loss in a 50-weight semi-synthetic reduces its effective viscosity to that of a 30-weight oil.
The Flow Factor
Ewing and Visser both compiled an impressive list of reasons to use a multi-weight oil year-round. Visser was characteristically blunt when he stated Our multi-grade flows better than anything on the market. Visser claims that at 20 to 30-degrees F, Shell multi-weight gets to the engines bearings three to four times faster after engine start up than would an appropriate single-weight oil.
We asked both experts to explain why the indicated oil temperature in our aircraft was lower with a multi-weight oil than it was with a single-weight oil. Ewing says that the improved flow rate and thermal conductivity of the multi-weight oil couples the piston ring belt to the cylinder wall better than a straight-weight oil does. In other words, the multi-weight allowed more of the engines heat to escape via the cylinder fins, instead of via the oil cooler.
Visser offered an alternative theory: He says the improved flow rate of the multi-weight oil allows better heat transfer between the engine parts, the oil and the oil cooler, a phenomenon noted in Pushy Galore, Bruce Bohannons tightly-cowled time-to-altitude record breaker. With a single-weight oil, Bohannons engine went over redline oil temps. With the multi-vis, pressures remained stable and oil temps below redline.
Shells Visser is adamant that corrosion in the engine is best controlled by flying often and not just for short, around-the-pattern hops. Visser says that Shells own tests show that with an indicated oil temperature of 170 to 180 degrees F, it takes an hour of flight to rid the crankcase of moisture generated during engine start and warm up.
While low oil temperature can be destructive, its not necessarily a good thing to get oil temperatures too high, either. Shells tests indicate that peak oil temperatures in an aircraft engine are usually 50 degrees F higher than the oil temperature indicated on the gauge. If the engine indicates over 220 degrees F with a properly calibrated oil temperature gauge, that means peak oil temperatures are approaching 270 degrees F, which is high enough to initiate thermal breakdown in mineral-base aircraft oils.
When asked about the apparent propensity of Lycoming engines to develop rusty camshafts, Visser believes this too is a moisture problem. The Lycoming cam is placed high in the engine block where it encounters a virtual humidity cabinet effect. When the engine is not run sufficiently long, or oil temperature is too low (below 170 degrees F), significant moisture can and does condense on the camshaft lobes.
Ewing and Visser are divided over the utility of using a stock, single-weight mineral oil (versus a multi-weight or (semi)synthetic) for engine break-in. Visser argues that its critical, for long-term corrosion control, to form a light varnish coat throughout the engine during engine break-in. Ewing is more circumspect, saying only that an owner shouldnt rely on varnish deposits for corrosion control. Note that Phillips manufactures a multi-weight oil, Phillips type M, that many consider the best oil for engine break in.
Visser says that the higher operating temperatures of the single-weight oil aid varnish formation and that, for this reason, a single-weight oil was preferable to a multi-weight oil for break-in. He makes an exception, however, for turbocharged engines. Using a multi-weight oil for break-in of a turbocharged engine can generate sufficiently high oil temperatures to form a protective varnish layer.
We asked Visser about reports that Shells 15W-50 multi-viscosity oil was generating elevated copper readings in oil analysis. Yes, he says, the EP additive used in Shells 15W-50 oil, tricresyl phosphate, had indeed leeched copper from engine surfaces. The EP additive used in Shell 15W-50, by the way, is chemically identical to Lycomings AD-mandated additive, LW-16702 for the 76 series engines.
Visser says Shell changed the additive package in 15W-50 during the spring of 1998 and that current shipments of 15W-50 should not cause elevated copper readings.
The manufacturers are unanimous in their technical preference for multi-weight oils over single-weight oils. While they almost grudgingly offer a single-weight product, its clear that their hearts are with the multi-weight as the better all-around oil. It appears to us that continuing support for single-weight oils is largely to satisfy customers who dont buy the company line on multi-weights.
The superiority of the multi-weight rests on two basic foundations: The first, and most important, is the vastly improved temperature performance of the multi-weight over the single-weight. Easier starting, better low-temperature lubrication and higher thermal stability all favor the multi-weight. Second, the manufacturers appear to devote greater resources, in general, to the additive packages in their multi-weight oils. This is largely because the multi-weight oils can, and do, demand a price premium over the single-weight offerings. The improved additive packages primarily help with extreme pressure (boundary) lubrication, corrosion control and detergency.
There doesnt appear to be any difference between single and multi-weight oils in basic lubricity. You shouldnt expect a multi-weight to reduce (or increase) engine internal wear rates. Our own tests have confirmed this by looking at controlled oil analysis samples from engines switching between single and multi-weight oils. We saw absolutely no difference in wear rates between single and multi-weight oils. Other variables, such as frequency of use and operating habits, may in fact influence wear more than oil choice.
Both Shell and Continentals position is that the problem of cylinder distress in big-displacement engines has been solved. Absent data to the contrary-which is very difficult to get due to the time spans involve in generating a good sample-we take this on face value for now.
Our opinion is that with the exception of corrosion protection (through varnish formation) during engine break in and a cheaper price, single-weight oils offer no advantage over multi-weight oils. On the other hand, multi-weight oils offer better low temperature flow characteristics, more sophisticated additive packages and better high-temperature stability than do single-weight oils. For cold weather starts, that alone may more than offset the price premium.
If your engine is past its infant hours, if it doesnt have TCMs big-displacement cylinder assemblies built between 1990 and 1998 and youre not cheaper than scrooge, we see no reason not to stick with a multi-weight oil.
Also With This Article
Click here to view “TCM Jugs and Multi-Vis.”
by Gregory Travis
Gregory Travis is a freelance writer and Cessna 172 owner. His Web site contains information about Lycoming engines. (www.prime-mover.org)