Synthetic oils and leaded fuel: Not a good combination

I received an interesting letter from Karl Sieg concerning my article on synthetic oil in the March 11 issue (Synthetic oil: Is it right for your plane?). Several years ago, Karl’s cousin had run synthetic oil in his 1946 T-craft and “”the engine ran so well he had to retune it.”” Karl then tried some synthetic oil in his car and the mileage improved. He then went on to ask about the problems, litigation and FAA rulings regarding the use of full synthetic oils in aircraft engines.


I received an interesting letter from Karl Sieg concerning my article on synthetic oil in the March 11 issue (Synthetic oil: Is it right for your plane?). Several years ago, Karl’s cousin had run synthetic oil in his 1946 T-craft and “”the engine ran so well he had to retune it.”” Karl then tried some synthetic oil in his car and the mileage improved. He then went on to ask about the problems, litigation and FAA rulings regarding the use of full synthetic oils in aircraft engines.

Lubricants made with synthetic base oils are excellent lubricants. The problem is that synthetics are so pure that they are very poor solvents. The automotive industry has developed additive chemistry that works with the synthetic base oil to provide the cleanliness characteristics needed to maintain a clean engine. This means that if you use synthetic-based oil in your car and change it at the proper intervals, your engine should remain relatively clean and offer excellent service and life. Major problems can occur if an owner decides that since the synthetic oil is more expensive, it should last longer and they then extend their oil change intervals significantly. In these cases the oil does not wear out, but can become completely saturated with contaminates like carbon, and then excess deposits can build up.

The problem with the use of synthetics in aircraft is the leaded fuel. Synthetic-base oils are so pure that they do not absorb the lead byproducts of combustion as the mineral base oils do. In addition, because of the necessary limitations on additive chemistry in the piston engine oil specifications, many of the additives used in automotive oils can not be used in aircraft oils.

The problem is not universal. When Shell Oil started to test synthetic piston engine oils back in the 1960s, many engines performed very well on the product. Unfortunately, numerous engines, usually the larger turbocharged opposed engines, started to show signs of increased oil consumption as early as 600 hours. When these engines were disassembled, the pistons looked like someone had taken a gray epoxy and coated the entire piston ring belt and glued the rings into the piston. In addition, the props were full of the same gray sludge, which was found to be lead byproducts of combustion. Shell never marketed a full synthetic oil, however, several companies did and found out the hard way that synthetic oils do not absorb the lead byproducts of combustion.

Aviation piston engine oils are approved against a SAE/Mil specification. The full synthetic oils that were marketed met that specification. When field problems occurred, the oil companies worked with the FAA to solve the problems. However, the FAA did not rule against synthetic oils, the oil companies removed them from the market.

In real estate, the three most important features for any piece of property are location, location and location. In aviation products, the three most important features of any product are no problems, no problems and no problems.

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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