Synthetic oil: Is it right for your plane?

One of the big questions in aviation is, “”What is synthetic oil and should I use it in my airplane?”" As always, my answer is a definite maybe.

One of the big questions in aviation is, “”What is synthetic oil and should I use it in my airplane?”" As always, my answer is a definite maybe.

The dictionary defines a synthetic as “”produced by synthesis; specifically, produced by chemical synthesis, rather than of natural origin.”"

In lubricants there are two different types of synthetic oils. The first is a synthetic di-ester, which is a reaction product of an acid and an alcohol. This is pure chemical base oil and is used in almost all jet engine oils. The big advantage of the di-esters is their super-high temperature and low-temperature characteristics. In jet engines it is possible to see instantaneous temperatures of up to 600?F, while the lubricant must flow at temperatures as low as 75?F. Two of the downsides of di-esters are their cost and the fact that they are very aggressive to normal seal material.

The other type of synthetic lubricant base oil is called synthetic hydrocarbons. There is some controversy in the lubricant world as to exactly what constitutes a synthetic hydrocarbon. As all hydrocarbon base oils come from crude oil, they need to be refined to improve cold-temperature flow and remove unwanted naturally occurring contaminates. So how much refining is necessary to call the oil a synthetic? Because of space considerations, I’ll limit this discussion to only poly alpha olephins or PAOs.

Back in the 1960s, Shell ran several flight evaluations of oil formulated with all-PAO base oil. The oil performed very well except in large engines, (turbocharged 520s and 540s). In some of these engines, the oil failed to absorb the lead salts from the combustion process. This resulted in gray sludge buildup in the ring belt and props. Shell then changed to a semi-synthetic containing only 50% synthetic base oil and the rest normal mineral oil to alleviate the problem.

At the present time, both Shell and Exxon offer a semi-synthetic multi-grade (MG) oil. (The Phillips MG is formulated with all mineral oil base stocks). The original question was, “”do these oils offer a real advantage in an aircraft?”"

The answer really depends on what you fly and where you fly. The advantage to the semi-synthetic oils is the improved low-temperature pumpability, and the high-temperature stability. If you live in a cold climate and have the opportunity to start your aircraft at temperatures below, say, 40?F (especially if preheating is not always available), then the oils have a definite advantage. You also can consider using a MG in the winter and straight grade oil in the summer or you can use the MG year around.

Semi-synthetic oils also offer an advantage in aircraft that are run very hard and hot. For example, many aerobatic and race pilots use a semi-synthetic oil as they usually reduce engine oil temperature and seem to provide better wear protection at extreme temperatures.

There are a lot of other smaller differences, such as that MG oils tend to reduce fuel consumption by a few percent and they tend to leak better than straight weight oils.

The bottom line is there is no one right answer for everyone. It is like airplanes: if there was one “”right plane”" for everyone, we would all be flying the same model, and that would be a very boring world.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at

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