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Air-cooled aircraft engines are twitchy, heat-sensitive machines that create wide variations in wear depending on the type of aircraft they are installed on, the type of cylinders employed, and how they are operated. They do not wear differently due to a particular brand of oil. There is a correct grade of oil, depending on how and where you operate your engine, but there is no correct brand.

The only type of oil you can safely use in an air-cooled aircraft engine is an aircraft-use oil. To use any other type of oil is to invite premature failure of the engine due to detonation.

Air-cooled aircraft engines are typically engineered to run on oils that are SAE 50 in viscosity @ 210 F, which is in the neighborhood of your engine oil operating temperature at cruise. W100 is an SAE 50 oil at operating temperature. So are 15W/50 and 20W/50. Any of these oils will work well in your engine. The multi-grades will allow your engine to spin over more easily at ambient temperatures - i.e., when cold since they are really SAE 15 and SAE 20 oils when cold. They have long chain polymers (viscosity improvers) that cause them to be more viscous at higher temperatures. But at operating temperature, all these oils will work well in your engines because they are SAE 50 oils at operating temperature.

What about dispersants vs. mineral oils? All aircraft-use engine oils on the market today (that we know of) are mineral oils, i.e., refined, petroleum-based oils. Some of them have an additive in them to aid in scavenging debris and carrying it to the filter or screen. These are called ashless dispersant (AD) oils. Without the additive, they are called mineral oils.

Tradition would have you using 100 mineral oil during wear-in of a new or overhauled engine, and then changing to an AD oil after two or three oil changes. We agree with this wear-in process. We don't see any significant differences in engine wear for running either of these types (mineral vs. AD oils) over the period of their useful lives. Perhaps the engine rebuilders do. If they do, we haven't read any definitive statements from any of them suggesting that one type of oil produces better wear results than any other.

We have a database of the wear produced from just about all types of aircraft engines. As an experiment, we compared the wear metals produced in a Lycoming IO-360 on four types of oils: Aeroshell W100, Aeroshell 15W/50, Exxon Elite 20W/50, and Phillips 20W/50. The sample group contained 571 IO-360s. Here are the results:

Aluminum wear ranged from 6-8 ppm, or no significant difference. Chrome ranged from 4-6 ppm, or no significant difference. Copper ran from 5-9 ppm; again, not much difference. Tin was 1 or 2 ppm for all types. Nickel ran at 2 ppm for all oil types. We found minor variations in iron wear ranging from 23 to 27 ppm. If you had 4 ppm of any of these metals in your eye, you would never know it.

But wait! Iron wear tends to increase the longer the oil is in service. So we ran another column on the spreadsheet, dividing iron wear by average hours oil use. Once again, we found no significant difference in iron wear.

Our conclusion from this (and other tests we have run using wear data) is, it does not make any difference brand of oil you use in your aircraft engine. There may be a correct grade. There is not a correct brand.

There are many variables to consider in how an aircraft engine wears. We consider the oil type to be the least of these variables (if it has any significance at all).

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