Airframe and Powerplant

All the Oily Facts

January 1, 2007

Why your engine loves its lube

In his classic aviation novel Fate Is the Hunter, author Ernest K. Gann writes about serendipity — stepping out of his routine to follow a hunch — as the reason he was able to spot a river of engine oil boiling out of his airplane's oil tank filler port. He saved the engine by quickly returning to his takeoff airport. This sparse recital of Gann's well-told adventure is sufficient to illustrate that airplane engines won't reciprocate for long without an adequate supply of lubricating oil.

Oily questions

Oil is the fulcrum that safe and reliable piston-engine operation pivots on. Ideally, reciprocating-engine oils should have the proper body (viscosity), high anti-friction qualities, maximum fluidity at low temperatures, minimum changes to viscosity with changes in temperature, high anti-wear properties, maximum cooling ability, and maximum resistance to oxidation. They should be noncorrosive as well.

To best achieve these targets, today's oils are a highly sophisticated mix of refined 60- to 100 million-year-old crude-oil stocks and relatively new additive packages.

The variety of reciprocating-engine oil choices may seem bewildering at first. There are semisynthetic oils and mineral-based oils. Mineral-based oils come in two varieties: noncompounded — often referred to as "break-in" oil — and ashless-dispersant (AD).

Both noncompounded and AD oils are available in single-viscosity or multiviscosity grades. Semisynthetic oils are always multiviscosity and, like all multiviscosity grades, are identified with labels such as 15W-50 or 20W-50.

These numbers indicate the viscosity (the oil's resistance to flow, with low numbers signifying a thinner, or easier flowing, oil) of the oil at ambient air temperatures and at engine operating temperatures.

Single-viscosity oil such as Aero-Shell 100 (SAE 50 weight) will flow like water when it's warmed to 180 degrees Fahrenheit during operation but will thicken to a honeylike syrup at 30 degrees F. This resistance to flow is critical during engine start because the thick oil is slow to circulate throughout the engine. One cold-weather solution is to change to lower-viscosity oil, such as AeroShell 65 (SAE 30 weight).

Instead of changing from one single-viscosity oil to another with the seasonal changes, pilots often choose to use a multigrade oil, which flows like 15- or 20-weight oil during cold-weather engine starts and then magically changes itself to 50 weight or even 60 weight when the engine is up to operating temperature. Multigrade oils cost a bit more than straight-weight oils but are indispensable in some climates.

To further muddy the waters, pilots need to understand aviation single-viscosity oil labeling. For instance, 50-weight oil is the thickest oil specified by Lycoming and Continental for their engines, but it's called "100," as in AeroShell 100. Forty-weight oil is thinner than 50 weight; it's called "80" in labeling parlance. To further gum up the works, 30 weight is called "65." Still with me?

The numbers

The numbers break down like this: 60, 50, 40, and 30 numbers are viscosity numbers from a Society of Automotive Engineers (SAE) test protocol. This test uses a Saybolt viscosimeter. The time in seconds it takes for a 60-cubic-centimeter oil sample to pass through a calibrated hole in the bottom of the heated cylinder determines the oil's viscosity and SAE number. The SAE viscosity numbers don't signify that two different brands of oil with an SAE 50 viscosity rating will have identical performance throughout the entire range of engine operating temperatures; the numbers merely mean that when the samples were both heated to the same temperature (210 degrees F) they flowed out of the viscosimeter in more than 80 seconds and less than 105 seconds.

Having pointed out one variable, it's important to know that oil brands made to the same standard — obsolete numbers are Mil-L-22851D and Mil-L-6082E; now the standards are SAE J-1899 and SAE J-1966 — are intermixable. Pilots needn't worry if they get stuck in the hinterlands for the night and discover that the oil level is down a couple of quarts during the morning preflight. Virtually all aviation oils sold today at local FBOs conform to SAE J-1899 (AD) or SAE J-1966 (noncompounded). Single-weight and multigrade oils also can be mixed without any problem, as long as the specification numbers match and the single weight is the correct viscosity for the operating conditions.

Noncompounded oils

Noncompounded oils can be thought of as "pretty good" oils when compared with additive-strengthened AD oils, and they have a special place in engine operations. Often referred to as "break-in" or "run-in" oils, these oils are used for the first 10, 15, or 25 hours — this interval depends on the engine manufacturer's or engine rebuilder's recommendations — after a new or rebuilt engine is put into service, or after a new or rebuilt cylinder or cylinders have been installed. Of course, like every rule in aviation, exceptions apply. For instance, Lycoming requires that its turbocharged engines be broken in on AD oil. Here's why:

Mike Caldera, Lycoming's manager of product support, explained that it's critical to maintain high engine-power settings during engine or cylinder break-in. Turbocharged engines don't suffer from manifold pressure drop-offs during climbs to, and while flying at, typical light-airplane cruising altitudes. Therefore, the turbocharger-equipped engines can maintain the high power settings — typically 70 to 75 percent — required to ensure a quick break-in. Logic dictates that AD oil also could be used to break in normally aspirated engines if, and only if, the pilot commits himself to never flying higher than 6,000 feet msl — the altitude above which normally aspirated engines can no longer maintain 75-percent power — during the first 50 hours of operation. That's an unreasonable operational restriction for most pilots, so noncompounded oil is used for break-in.

The worst thing a pilot can do when breaking in an engine or cylinder is to baby the engine, meaning to use lower power settings. Here's why:

When spark plugs fire at the end of the compression stroke, a chain reaction is started in the fuel/air mixture that rapidly increases the volume of high-pressure gases within the cylinder combustion chamber. A portion of that gas pressure seeps down the sides of the piston and is directed into the space between the back of the top two rings — called "compression rings" — and their ring grooves. These gases serve to push the ring out against the cylinder wall, tightening the ring-to-cylinder wall seal.

New and rebuilt cylinder walls are ground to what's called a "ring finish," consisting of a tight-knit series of microscopic valleys and peaks. This surface and the face of the reciprocating compression rings don't seal especially well until wear scrubs the "peaks" off, resulting in a plateau-and-valley profile. To speed along the wear process required for a tight seal, noncompounded oil is used.

Oil companies — including Aero-Shell, Exxon, Castrol, and Phillips 66 — all produce noncompounded oils. Exxon and Phillips 66 produce noncompounded multigrade oils.

Ashless and dispersant

The A in AD oil means the oil contains additives that actively clean the inside of the engine. These agents differ from automotive detergents in that these additives contain no metals or other compounds that could form ash deposits when burned. It's been proven that combustion-chamber ash deposits lead to preignition, which can progress into engine-damaging runaway detonation.

Additive packages also contain anti-oxidants and anti-foaming agents. Multiviscosity oils also contain viscosity-index improvers, which help to stabilize the oil performance across a wide range of operating temperatures. The D in AD stands for dispersants that hold the wear metals and other contaminants picked up by the oil in suspension so they can be picked up by filters and drained out during oil changes. Noncompounded oils don't contain dispersants. In the past, mechanics believed that adding AD oil to an engine that has been operated for hundreds of hours on a steady diet of noncompounded oil was a recipe for disaster. However, information from Exxon and Castrol contradicts that old airport tale. In spite of these findings, adding AD oil to a mineral-oil-serviced engine is still frowned on by mechanics in the field because of the belief that the AD oil will immediately start picking up and circulating the crud that's accumulated in the sump.

Snake oil or miracle cure?

Lycoming introduced the 160-horsepower O-320-H2AD engine in the mid-1970s. Cessna removed the tried-and-true 150-horsepower O-320-E2D from the engine compartment of its best-selling 172 and stuck in Lycoming's new powerplant beginning in 1977. Unfortunately the engine was not ready for prime time and more than one airplane had to be landed short of the destination airport during the initial delivery flight because of metal failure of the camshaft and lifters. This was because of extremely high camshaft-to-lifter pressures. Lycoming eventually succeeded in making a long-lasting fix and, given the right conditions, the -H2AD engine has proven to be durable. One part of the fix was an airworthiness directive (AD 80-04-03 R2 and Lycoming service instruction 1014M) that mandated the addition of a pint of an anti-scuffing, extreme-pressure (EP) additive (Lycoming part number LW-16702) at each 50-hour-interval oil change.

LW-16702 prevents scuffing by chemically combining with iron to form a phosphate layer on the part surface. This layer is extremely thin and is swept away by shearing friction — which occurs every time the lobes on the camshaft contact the hydraulic lifter body. The phosphate layer chemically renews itself in an instant. Lubricant producers soon realized that increased camshaft-to-lifter protection is highly desirable and started adding an extreme-pressure agent like LW-16702 to their engine oils. AeroShell 15W-50, W80 Plus, and W100 Plus, Exxon Elite 20W-50, and Phillips Type A 100AW oils all contain this additive. Because the anti-scuffing additive is mixed in these oils, use of any one complies with the requirements of the AD and eliminates the need to add a separate pint of LW-16702 at each oil change. There's one important caveat governing the use of these oils containing the EP additive. Engine Components Inc. (ECi) — the San Antonio company that supplied a new set of its Nickel+Carbide cylinders for AOPA's Win a Six in '06 Sweepstakes aircraft — warns pilots and engine builders that the same extreme-pressure safeguards that work so well at protecting internal parts after an engine is broken in will also slow or prevent the wear required for good cylinder break-in.

To recap

Unless otherwise directed by the engine or cylinder manufacturer, use mineral oil during new-engine or new- or rebuilt-cylinder break-in. These include single-viscosity oils such as AeroShell 65, 80, 100, and 120, Castrol Aviator S65, S80, S100, and S120, and multigrade oils such as Phillips 66 Type M 20W-50 and Exxon aviation oil 20W-50.

Maintain 65- to 75-percent power settings and continue to use mineral oil until the oil usage stabilizes, but not for more than 50 hours.

After 50 hours, switch to an AD oil and change the oil at 25 hours if the engine does not have a filter installed and at 50-hour intervals if it does.

The manufacturers say...

Actually, with the exception of the above-noted caveats, the engine manufacturers recommend mineral oil during break-in and AD oil for the life of the engine, unless a cylinder has been changed. The Columbia 300 pilot's operating handbook pretty well sums this up. It specifies the use of SAE 30, 10W-30, 15W-50, or 20W-50 when outside air temperatures are below 40 degrees F. Above 40 degrees F, use SAE 50, 15W-50, 20W-50, or 20W-60. When asked about oils, a cross section of engine overhaul shops overwhelming said to follow the manufacturer's suggestions.

Single-viscosity oils — sans EP additive — are the least expensive aviation engine oils. Single-viscosity oils with the EP additive cost an average of 20 percent more, and multiviscosity semisynthetics, which all contain the EP additive. cost at least 35 percent more. Over the course of a 100-hour flying year, the difference in cost is not very large. Cost is secondary to the requirement of making sure that the correct-viscosity oil is used. Some AOPA members choose to save a few dollars by using single-viscosity oil during warm summer months and multiviscosity oil during cold winter months.

In some regions of the country, AeroShell is everywhere; in others Phillips 66 oils are more widely distributed. These variables must always be taken into account, but there are two rules that should always be adhered to. Unless you're breaking in a turbocharged Lycoming engine, it's best to use noncompounded oil during break-in of an engine or cylinder; and always change your oil and filter at four-month intervals, or at the appropriate hours flown, whichever comes first.

E-mail the author at steve.ells@aopa.org.