Bruno Defelippe from Paraguay is confused about mineral oils.
“Aren’t all engine oils ‘mineral oils’?” he asks. “What is the difference with ‘non mineral oils’ — if those exist.”
Like so many things in aviation — and life — the answer is “well, kind of, sort of, yes.” All engine oils are mineral oils.
The problem here is that every industry, region of the country, and profession have their own language and terminology.
Many years ago, I gave a talk to a group that included several medical doctors. After the talk, one of them commented that he had a new engine in his airplane and he did not know that he could go to the hospital pharmacy and just get some mineral oil there for his engine.
After I settled down some, I explained that mineral oil in general aviation refers to a product that meets the Mil-L-6082E/SAE 1966 specification for aviation oils.
Usually used for the break-in process for some aircraft piston engines, it is basically just mineral base stocks with almost no additives (except for a small amount of antioxidant and pour point depressant).
The theory behind the use of mineral oil for break-in of new or overhauled engines is that with no ashless dispersant cleanliness additives, more of the metal wear particles from the new parts stay in the ring belt area and act as lapping compounds to wear in the new cylinders and ring sets.
The other classification for aviation piston engine oils is the Mil-L-22851D/SAE 1899 specification for oils that are normally referred to as AD or ashless dispersant oil. These products are generally just the mineral oil products with the addition of an ashless dispersant for improved cleanliness.
The exception here are the semi or partial synthetic multigrade oils that are blended with some non-mineral oil base stocks.
Many people mistakenly call AD oils detergent oils — and that sort of works in the aviation community where there are only the two oil specifications for certified aircraft piston engine oils.
But it is incorrect when dealing with other classifications of oils. In the lubricant world, a detergent oil is one that contains an ash type or metallic based detergent as opposed to an ashless dispersant. The difference sounds small, but there is a significant difference in the engine.
In an aircraft engine, an ash type detergent oil will build up deposits in the engine, which will probably lead to pre-ignition, and will probably destroy the engine.
What Oil Should I Use to Break In My Aircraft’s Engine?
So what oil should you use to break in a new engine? The answer varies depending on what company manufactured or rebuilt your engine and that company’s recommendations.
For example, Continental recommends break-in using a mineral oil that meets the Mil-L-6082/SAE 1966 specification in all its engines. But Lycoming recommends using an AD oil meeting Mil-L-22851/SAE 1899 specification for break-in in its turbo-charged engines and mineral oil for break-in of most non-turbo piston engines. There are also some different recommendations for certain models, like the O-320H series of engines from Lycoming.
Additionally, several engine rebuild shops and cylinder rebuild shops have their own recommendations.
So, what is the bottom line on oil recommendations?
Always check with the people who built your engine on their recommendations for break-in and then follow those recommendations closely.
Also ask for their recommendations on which oil to use after break-in and the recommended oil change intervals.
Only — and I do mean only — use oils that meet the Mil/SAE specs recommended.
Last tip: Always monitor and adjust your oil temperature to get it up to around 180°F or so during cruise. As oil goes through the engine, it will get up to 50° hotter than indicated. If your temperature is well below 180° you will not boil off the water and if it is well above 180° you can cause coking of the oil.
