Updated: Dec 6, 2020
The key to any lubrication system is the ability of the oil to protect moving parts and to prevent wear. In an aircraft piston engine it is imperative that the oil reaches the engine’s vital parts as soon as possible to prevent excessive wear. Operation beyond acceptable oil temperature limits or failing to adequately pre-heat the engine could possibly lead to premature overhaul.
The four main functions of the engine oil are lubrication, sealing, cooling and cleaning. Other relevant functions include corrosion protection and hydraulic action (for lifters or constant speed propeller). The oil must be clean and of the proper viscosity to do all of these jobs well. Understanding the properties of the oil inevitably leads to a better understanding of your aircraft.
The viscosity of a liquid is a measure of its resistance to flow. The basic theory of viscosity suggests that high viscosity (thick – more viscous) oil flows more slowly and that low viscosity (thin – less viscous) oil flows faster. There is also a significant relationship between viscosity and temperature characteristics of the oil. Cold oil will remain more viscous and resistant to flow unless the oil is adequately pre-heated. When the oil becomes hot through normal engine operation, it flows more easily and performs as expected. These characteristics are often referred to as the weight of the oil and can become critical to optimal engine operation.
All general aviation oils are petroleum based and can also be formulated with additives that serve very specific functions. The most basic formula is straight mineral oil without any special cleaning additives or inhibitors. Mineral oil is still commonly used as engine “break-in” oil and usually marketed as “Type M” aviation oil. Detergent oils were introduced many years ago with some primitive cleaning properties but were eventually phased out as the resulting deposits caused localized hot spots within the engine cylinders. A more successful approach to improving the qualities of the basic mineral oil came in the form of Ashless-Dispersant oil also known as “AD oil”. This scientifically improved additive package was no longer prone to leaving deposits and even excelled at holding other contaminants in suspension until drained out at oil change. Further improvements to these petroleum products over the years led to semi-synthetic blends made from a combination of petroleum based and synthetic components like the widely used Aeroshell 15W50. Mineral and AD oil remain as the two types accepted for small aircraft.
Aircraft piston engine oil grade designations are based upon a general manufacturing standard known as SAE (Society of Automotive Engineers) referencing the viscosity of refined oil varieties. A designated SAE number determines the oil grade advertised on the bottle as double the SAE value. Examples of these numbers are detailed in the following chart (referencing single grade oils only).
Shell oil products used in aviation are known as Aeroshell and the W identifier designates an AD oil grade. No prefix to the grade number designates a Mineral oil of the same grade. Other brands of oil will use a similar format with prefixes and multi-grade varieties. The lower grades represent a (thinner) winter oil and the higher grades more suitable as a (thicker) summer oil. The more extreme grades 65 and 120 are not as common now since oil formulation has greatly improved in recent years narrowing down the seasonal choices to 80 and 100. There are also other specialty oils available primarily for radial type engines and others used as engine preservatives during periods of extended inactivity.
The introduction of multi-grade oils, also known as variable-viscosity oils allow operators to use the same oil year round rather than switching single grade products between cold and warm climates. Multi-grades are simply AD oils designed to self-compensate as a summer or winter oil according to temperature changes. The Aeroshell 15W50 designator advertises SAE 15 properties for winter and SAE 50 for summer. Another example would be the Phillips 20w50 X/C originally promoted under the more traditional Phillips 66 brand.
While multi-grade oil products may appear to be the obvious choice, always consult with the aircraft owner’s manual for proper oil recommendations. Some engines simply have better results when operating with single grade oils whereas others may specify multi-grade only, otherwise the best choice is ultimately what you (or your mechanic) feel is best for the role of your specific aircraft.
The best operating considerations for your engine include frequent use, power management within specified operating parameters and pre-heat the engine/oil when necessary. Keep in mind that oil pressure indication may take longer in a cold environment because of the more viscous oil at start-up.
When adding oil to the engine, using the same brand and grade used at the oil change is always preferred but if you do not have that option, it is also permissible to use another brand or grade of oil depending upon what is available to you. All oil varieties are compatible by mil-spec (Military Specification) as either a Mineral specification or AD specification as shown below.
Many aircraft owners also take advantage of oil additives as a supplement to their regular oil or already blended in with their chosen brand. Many years ago, Lycoming introduced an oil additive (LW16702) as an anti-wear/anti-scuffing agent recommended for certain engine models although the additive would also be used in other Lycoming models for added engine protection. When Aeroshell introduced their popular 15W50 semi-synthetic multi-grade, it also included this same additive in the oil mixture itself and complied with the Lycoming AD for their specific models. The anti-scuff additive package has also been included in the single grade options advertised as W80 (or W100) Plus
Phillips also offers the LW16702 compliant formula in their 20W50 multi-grade advertised as AW 20W50 X/C Victory oil. The AW prefix represents the integral “Anti-Wear” formula. The mainstay Aeroshell and Phillips brands continue to dominate the piston engine oil market while other competing brands have mostly come and gone.
ASL Camguard is another popular FAA approved oil additive compatible with all mineral based, semi-synthetic and even fully synthetic piston engine applications. This additive offers multiple corrosion inhibitors for both ferrous and non-ferrous engine metals as well as the usual wear protection properties. Many owners/ operators will gladly discuss their engine oil and/or additive preferences while eagerly supporting the reasons behind their choice