Engines depend on a thin film of oil to eliminate sliding friction and wear associated with metal-to-metal contact between moving engine surfaces. For maximum engine life, this thin film of lubricant must be clean and free of contaminants. Engine oil contaminants originate from several places, including combustion blow by (raw fuel + exhaust gases), lead byproducts from the use of avgas, outside air ingestion associated with the engine air induction system, and engine wear particles. Since contaminants accumulate during engine operation, certified engine manufacturers recommend that oil should be changed every 25 operating hours in an engine not using an oil filter, and 50 hours when an oil filter is used, or in either situation every four months. For many general aviation engines that are flown infrequently and operated 50 or less hours per year, the four-month limit is a more important criteria than the 25- or 50-hour limit. The four-month limit removes water which can condense within an engine during periods of inactivity and from not achieving an engine oil operating temperature around 180 F.
As recognized by various engine rebuilders, there is a distinct advantage associated with the use of a 100 percentmineral oil (such as Phillips 66 X/C Aviation Oil) versus semi-synthetic aviation engine oils. This advantage is associated with the superior solvency of mineral versus a semi-synthetic oil. The greater solvency eliminates the accumulation of lead byproducts and keeps contaminants in suspension, allowing these contaminants and lead byproducts to be removed from the engine during the oil change.
Mineral oils with a greater solvency provide for a much cleaner engine with better heat transfer, better combustion control, and less sludge and carbon.
Full and semi-synthetic oils are proven to be effective in car and truck engines, but the over road engine oils are not exposed to the lead byproducts associated with the use of avgas containing tetraethyl lead (TEL). Plus, over road engine oils contain cleanliness additives that are not present in aviation oils.
The key differences between “straight grade” and “multi-viscosity” oils
Viscosity is the measure of a fluid’s resistance to flow. It is the single most important property of any lubricant as viscosity influences the oil’s ability to keep moving metal surfaces separated during high and low temperature operation. It also affects the oil’s ability to circulate quickly throughout an engine during start up.
The problem with straight grade aviation engine oils is that the correct viscosity grade for a normal operating temperature is often too viscous to flow easily at start up, and the correct viscosity grade for easy flow at start up is not thick enough at normal operating temperatures. This shortcoming is greatly reduced with multi-viscosity oil, which has been the standard for virtually all car and truck engines for decades.
As shown on the SAE viscosity chart, relative to straight grade oils, which are only required to satisfy the high temperature specifications, multi-viscosity oils meet both the high- and low-temperature specifications of SAE J300. The “W” rating, which stands for “winter,” indicates that the oil has met the requirements of low-temperature tests measuring both cold flow and cold cranking ability. The smaller the number that precedes the “W,” the lower the temperature at which it was tested.
Meeting both the high and low temperature specifications ensures a multi-viscosity oil flows easily at low temperatures while maintaining sufficient viscosity and film strength at high operating temperatures. This benefit allows a multi-viscosity oil to flow throughout the engine quickly during start up to achieve full lubricant film separation between moving surfaces while still protecting the engine during operating temperatures. Both flow properties are important to the longevity of an engine.
Unlike the mobile equipment applications, such as automotive, trucking, or heavy-duty off-road equipment, which has been using multi-viscosity oils for decades, only about half of today's aviation industry uses multi-viscosity oil. Many in the aviation community cite two reasons for not using a multi-viscosity oil: either (1) they live in a warm climate and consequently have no need for the low temperature flow properties provided by a multi-viscosity oil, or (2) there is a consistent perception that straight grade oils adhere to engine parts better than their multi-grade counterparts, thus providing better protection during periods of inactivity.
As shown in the table below, even at 60 F a multi-viscosity oil is approximately 40 percent less viscous than a straight grade oil, allowing the lubricant to flow more freely throughout the engine during start up. This viscosity difference becomes even more significant at temperatures below 60 F. At elevated temperatures, such as those in the ring belt zone, multi-viscosity oils are slightly thicker than straight grade oils.
Intuitively, one would suspect that multi-viscosity oil would more completely drain off idle engine components since a multi-viscosity oil is thinner than a straight grade oil at ambient temperatures. However, Phillips 66 Lubricants laboratory testing has definitively shown that all aviation engine oils drain off the parts down to a residual oil film thickness of 3 to 4 microns (25 microns = 1/1,000 of an inch) after just a few hours – regardless of whether the oil is a multi-viscosity or a straight grade oil.
Technical differences between ashless dispersant vs. non-dispersant oils, and recommendations for break-in of new and/or overhauled piston engines
The first aviation oils were simply base oils made to meet the requirements for various viscosity grades. These early oils were made without any performance-enhancing additives of any kind. The performance requirements for these early aviation engine oils were covered in military specification MIL-L-6082. This specification defined a non-additized engine oil and included multiple basic performance and chemical tests as well as cleanliness and ash limits.
Additives like antioxidants and ashless dispersants weren’t used in aviation engine oils until the specification MIL-L-22851 was introduced in June 1961, representing the first major evolution in aviation piston engine oils. Antioxidants (sometime referred to as oxidation inhibitors) enhance the oil’s ability to resist oxidation allowing the oil to retain its lubricating properties longer. Dispersants are non-metallic additives that help keep sludge, varnish, and other microscopic contaminant particles in suspension preventing these contaminants from settling out within the engine, allowing them to be removed from an engine during an oil change. Dispersants are similar to detergents, except they are able to keep larger quantities in suspension and are ashless when burned, providing a much cleaner engine combustion chamber.
The military specification MIL-L-6082 was replaced by Society of Automotive Engineers (SAE) Standard J1966. Since these products lack the additives needed to protect an engine, oils meeting the J1966 specification are generally used for engine break-in. These oils are often inaccurately referenced as “mineral oils.” Technically speaking, mineral oils are manufactured by refining crude petroleum to remove impurities in the crude petroleum and to establish the various viscosity grades needed for lubrication.
Eventually, MIL-L-22851 was replaced by SAE Standard J1899. Since these products contain additives needed to protect an engine, J1899 oils are used during the operational period of an engine. Many in the general aviation community erroneously believe that J1899 oil cannot be used for engine break-in. This is not true. Many J1899 oils, such as Phillips 66 X/C Aviation Oil, can be used as a break-in oil as well as an operational oil. Phillips 66 Lubricants believes that oils containing ashless dispersants offer an advantage during break-in, since the break-in process causes an increase in blow-by gases and break-in wear metals. Since this is the dirtiest time for an engine, an ashless dispersant oil will suspend the contaminants and keep them from being deposited inside the engine.
Rust prevention and engine storage recommendations
Aircraft engines are designed to be used on a regular basis, so any length of inactivity must be met with proper maintenance actions to ensure the engine remains in proper working order. In some cases, rust can occur because the engines aren’t flown long or hot enough to evaporate the water generated from the fuel combustion process or from improper use or installation of air/oil separators. The most common cause for rust is improper aircraft storage.
As developed by Harold Tucker, former director of technical information for Phillips 66 Lubricants, methods to prevent rust vary depending upon the length of inactivity. Preservative oils, such as Phillips 66 Aviation Antirust Oil, protect aircraft piston engines from rust in a manner that standard aviation engine oils do not. However, these products are not suitable for extended flight hours since they don’t contain an ashless dispersant additive that protects engines during normal use.
If your plane is flown several times a month, for periods longer than one hour, there is little, if any, need for additional rust protection. For infrequently flown planes, such as one that sits for several weeks to a month or more, we recommend either reducing the four-month drain interval suggested by the engine manufacturer or maintaining the suggested interval and adding a 10 percent concentration of Phillips 66 Aviation Antirust Oil 20W50 to the engine oil to boost the rust protection provided by conventional aviation oil. For long-term storage, such as more than three months or more, the engine should be filled to the normal operating capacity with full change of preservative oil, and then the plane should be flown for at least 30 minutes to fully circulate the oil. In addition, take all other steps for long periods of storage, such as covering the exhaust.
Steven Strollo has been employed in the lubrication field for 17 years and is Phillips 66 Lubricants primary contact for aviation engine oil questions. He has hosted aviation engine oil forums at Sun ‘n’ Fun and EAA AirVenture for the last 10 years, and frequently provides FAA Inspection Authorization refresher training. Strollo maintains Certified Lubrication Specialist (CLS) and Oil Monitoring Analyst levels I and II (OMA I and II) certifications from the Society of Tribologists and Lubrication Engineers.
