Are there synthetic oils available for aircraft engines?

Today’s question comes from Verd Erickson, who asked “”Are there any synthetic oils available for aircraft engines?”” His letter was actually sent to Paul McBride, GAN’s engines expert, but ended up with me.


Today’s question comes from Verd Erickson, who asked “”Are there any synthetic oils available for aircraft engines?”” His letter was actually sent to Paul McBride, GAN’s engines expert, but ended up with me.

I could answer this question with yes, no or just kind ‘a sorta. First, the yes part. Almost all turbine engine oils are fully synthetic. There are a few really old engines and some ex-Soviet bloc aircraft that operated on mineral-based turbine oils, but almost all of the modern U.S.-built jet and turboprop engines operate exclusively on synthetic oils. It is very critical that only the properly approved oil be used in these aircraft. Turbine engines operate at very high temperatures and a mineral-based oil would crack and turn to carbon very quickly.

The no part of the answer is for the piston engine aircraft market. There are no full synthetic oils qualified for certified aircraft piston engines. The reason for this is leaded fuels. Back in the 1960s, Shell developed a full synthetic-based aircraft piston engine oil. The oil performed very well in lab and bench engine tests. This oil was then put into a variety of different aircraft for flight evaluation. The oil was great, with low consumption, lower than normal wear metal results, and unbelievable low temperature starting capabilities. But ? and there is almost always a but ? the long-term cleanliness of the product was very poor in larger opposed piston engines. Many of these engines ran perfectly for 600 to 900 hours. Then the oil consumption started to go up. There were engines that went from six to eight hours per quart to one to two quarts per hour. When these engines were torn down, the pistons looked like someone had taken gray epoxy cement and glued all of the rings into the piston. When the deposits were analyzed, we found out that the “”glue”” was lead by-products of combustion. (The prop hubs also were full of this gray lead deposit material.) It was determined that in a piston engine, the lead by-products of combustion that impinge on the cylinder walls and piston crown area are carried away by the natural solvency of the base oil, not the ashless dispersant. Due to the purity of synthetic base oils, they are excellent lubricants, but poor solvents.

Several other companies actually marketed full synthetic aviation piston engine oil and experienced the same problem. Consequently, there are no full synthetic aviation piston engine oils on the market today. There are several semi-synthetic oils on the market. These oils contain a blend of part synthetic and part mineral oil base stocks. This yields an oil with improved low temperature flow characteristics and excellent high temperature stability ? yet it contains some mineral base oil to help carry the lead deposits away from the piston crown area.

The three most important criteria for an aviation product are: No problems, no problems, and no problems. Full synthetic based aviation piston engine oils are just another example of a product that performed great in the lab, but did not meet the needs of the real 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 Visser@GeneralAviationNews.com.

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