For oils, it’s all about the chemistry

Just about every month I get several questions concerning the difference between aviation and automotive engine oils. This month I received a question about using Aeroshell 2F in a vintage automobile and the ever popular question about why one should not use automotive or heavy duty engine oil in their aircraft engine.

The answers always go back to the chemistry of the oils. Automotive and heavy duty oils all contain ash or metallic additives and aviation piston engine oils are all ash less.

The auto oils contain zinc dithio phosphate as an antiwear additive and some form of ash-containing detergent, such as a calcium sulphate.

I know many people call Aeroshell W100 detergent oil but it is not, it is an ash less dispersant or AD oil.

The zinc additive greatly increases the load carrying capacity of an oil by chemically attaching itself to the metal parts. This coating is then sheared off in an action called sacrificial lubrication.

But because of its chemical action, it tends to destroy things like silver master rods in radial engines and copper alloy exhaust valve guides in flat engines.

Ash-type detergents clean up dirty engines and are very good at neutralizing acids due to their high base numbers. That is all well and good, but the negative part is that ash type detergents can build up metallic-containing deposits in the combustion chamber.

These deposits can start to glow under high load, like at takeoff and climb out. This can lead to a condition called deposit induced runaway surface ignition (DIRSI), which can mess up an engine in very short order.

I have never experienced DIRSI in an actual aircraft engine and I hope to never do that. But I have seen it in a lab engine and it is not good. The engine started hammering very badly during full throttle run and it sounded like a handful of rocks in a food blender.

We shut down the ignition and the noise changed, but continued. We closed the throttle, but the hammering continued until it ran out of fuel in the carburetor. When we pulled the engine down, it had a hole in one piston.

DIRSI is uncommon in auto engines because of liquid cooling, low load operation, and normally low oil consumption. But aircraft engines are more susceptible due to higher combustion chamber surface temperatures with air-cooling, higher oil consumption, and higher loads, especially during takeoff and climb out.

Back to the original questions: Most old cars that have been running modern day automotive oils would not benefit from the addition of Aeroshell 2F because the ash detergent does an excellent job of rust protection and acid neutralization.

The exception would be a really old car that has been run on only straight mineral oil. Here the 2F might help preserve the engine during long periods of storage.

The answer for the second question depends on how lucky you think you are. If you have a good tight engine with very low oil consumption on ALL cylinders, do not have any silver bearings or copper exhaust valve guides, do not worry about FAA regulations, and live a charmed life, then you may consider trying it.

But like the test engine, it doesn’t take much to result in DIRSI. And the most probable time for it to occur is after takeoff and during climb out.

If it would occur, the noise will scare the daylights out of you. You now have very limited options on what to do next. You can hope the engine will stay together long enough to get you back safely before coming apart, or you can chop the throttle, lean it way out, and look for a place to land.

I have heard of and read on the Internet about people who have gone to full TBO on automotive oils. This does not prove that it is safe to use. It just proves that they were lucky.

Just being lucky is not a good maintenance philosophy for aircraft owners and pilots.

Lubrication for aircraft needs to be based on using the approved oils, keeping the oil temperature in the right range, and changing the oil when recommended.