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Reprinted from AM/September 1998

There is no doubt that operating and maintaining an aircraft can be expensive, so private pilots often look for bargains in everything from an engine overhaul to a gallon of avgas.

At this year's Sun 'n Fun Fly-In, several pilots told us they were experimenting with using automotive oil in their aircraft because they can buy it at local discount outlets for about one-third the cost of aviation oil from an FBO. Another reason was that the aviation oil they preferred using wasn't always available at their local FBO.

As producers of both automotive and aviation oils, this concerns us for many reasons, but specifically because automotive oils are neither made for nor tested in aircraft engines.

Consider these factors before trying automotive oil in an airplane engine:

The additives in automobile oils may harm an aircraft engine. Detergents and anti-wear additives in automotive oils are prohibited in aviation oils because their ash content could create deposits that cause pre-ignition or even detonation in aircraft engines. Ash content is not restricted in automotive oils, and can range from one-half to one percent. Aviation oils, such as Phillips 66 X/C multi-viscosity oils and Type A 100AD single-grade oil, on the other hand, use ashless dispersants to suspend dirt in the oil, avoiding the risk of pre-ignition.

Structural and operational differences between aircraft and automotive engines mean oil must perform differently in each. Automotive oil additives are designed to work best within a range of temperatures and pressures at constantly changing power levels inside water-cooled auto engines. However, aircraft engines are air-cooled, and aviation oil additives must protect engines under completely different conditions. Other structural and operational factors also may affect oil performance: Automotive engines are made mostly of steel, while aviation engines are primarily made of lightweight alloys. Auto engines run at varying RPMs, while aircraft engines run at constant speeds. Automobiles experience stop-and-go operations, while aircraft engines, we hope, do not.

Aviation oils are made specifically to meet aviation engines' performance requirements. Aviation oil formulations do not change unless they are extensively tested and approved for use in aircraft engines. That means the aviation oil that met an airplane engine's needs 30 years ago will still meet those standards today, and that same oil will also meet the needs of a new aircraft engine.

Automotive oils are not uniform. The composition of all aviation oils is stringently regulated to avoid harming any aircraft engine components, so there is no problem with using different brands of approved aviation oils. Conversely, formulations for automotive oils may differ with each lubricant supplier. American Petroleum Institute (API) guidelines for automotive oils permit the use of different base oils, additives and viscosity modifiers even in the same brand of automotive oil. While automotive oils can meet all API standards, without approved standards for testing auto oils in aircraft engines, there is no way to tell what their effect will be in aviation use.

Automotive oil formulations change frequently, using different additives, base oils and viscosity modifiers that have been proven in automobile engines. These changes in automotive oil formulations are spurred by the development of new, high-performance auto engines. Without aviation engine testing, however, there is no proof that these oil formulations will perform acceptably in aviation engines.

Aircraft engines require higher viscosity than many auto oils provide. Automotive oils are getting lighter (less viscous) all the time.

So, while we understand an operator's desire to save money with ever-increasing aircraft operating costs, cutting corners on the lifeblood of the engine is not the way to do it.