Lubrication 101: Piston engine oil, its functions, types, and characteristics

Piston engine oil, its functions, types, and characteristics. By Barb Zuehlke Oil. Its basic functions within an engine include reducing friction, cooling, sealing, cleaning, and serving as protection for moving parts. But it's often...

Single or monograde
Monograde oil is a petroleum-based lubricant with a single viscosity grade. Single-grade oils are considered by some to be better for warmer temperatures, but can't provide the flow needed for cold start-ups without the use of a heated hangar or engine preheating devices. In some locations one viscosity can be used throughout the year.

There is a long-running debate between single and multigrade oils. Some users prefer the single grade as it provides better high temperature and shear stability. Others believe only a multigrade can deliver the performance required.

Straight mineral oil contains no dispersant and is usually recommended by manufacturers for the first 50-hour break-in period on new or newly overhauled engines. It allows faster piston ring seating and allows the accumulation of some advantageous deposits which lead to better oil loss control.

Multigrade oil
Multigrade oils are either full mineral-based oil or a synthetic blend. Multigrades are primarily designed around all-season operation and convenience. They meet the requirements of more than one SAE viscosity grade classification, and are therefore more suitable for use over a wider temperature range than a single-grade oil. Multigrade oils contain viscosity improvers that reduce the tendency of an oil to lose viscosity or thin out at different viscosities. Other benefits incude lower oil consumption and better fuel economy.

Ashless dispersant
These oils are represented by both multigrades and the AD monogrades and are governed by SAE J1899. Ashless dispersants are additives designed to minimize deposit formation. They do not contain metal compounds, which would contribute to deposits in the combustion chamber. The dispersants help prevent contaminants from forming sludge that could plug oil passageways. They help the oil suspend combustion by-products, keeping them dispersed until the oil is drained.

Synthetic oils are polyalphaolefins produced from chemical synthesis rather than the refinement of petroleum oils. Through the refining process molecules are manufactured to a uniform size and structure. Characteristics, although dependent on application, include better oxidation stability or resistance, higher viscosity index, a lower pour point, lower coefficient of friction, and longer life. One disadvantage is cost which can be several times higher than mineral-based oils.

Synthetic oils have been linked to issues with seal deterioration, as well as solubility issues with leaded fuel causing deposits to develop, and passageways, like arteries, getting clogged. Some maintenance technicians have even developed the skill of identifying the engine oil used by the level of buildup left behind. Only one fully synthetic aviation piston engine oil ever made it to market. This product was subsequently pulled for some of the reasons mentioned.

Oil quality is determined by refining processes but additives can improve overall performance. Additives, usually new technology brought to the market through multigrades, can include anticorrosion, high load, and anti-scuffing characteristics. They can dramatically improve the performance of lubricants in older technology engines.

Textron Lycoming developed an anti-wear/anti-scuffing additive LW-16702 outlined in AD 80-04-03 R2. Since its introduction into the market, several oils have received supplemental type certificates that can be used as alternatives.

So whether you choose a single-grade or a multigrade, straight or an ashless dispersant make sure the oil change intervals are maintained according to manufacturer recommendations. Also consider the aircraft type, engine type, and the flight profile to help determine the correct oil for the environment and application. This will help ensure the oil you're using performs to the standard required.

Additional Resources

Houston, TX

AirBP Lubricants
Parsippany, NJ

ConocoPhillips Lubricants
Houston, TX

Irving, TX

Society of Automotive Engineers
Warrendale, PA

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