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In one episode of the 'reality' show Airplane Repo a pilot topped off an aircraft engine with motor oil; I think it was a Cessna 182 but I may be wrong. What would be the effect on the engine? Is it a practical (and safe) alternative if aviation oil isn't available?

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By motor oil, I assume that you mean automotive motor oil. In that case, it's a bad idea for a bunch of reasons. First, every airplane POH I've seen has specified that the oil must qualify to a standard - either MIL-L-6082 or MIL-L-22851 (though those have been superseded by an SAE standard). Modern automotive oil doesn't meet that standard, and therefore is effectively forbidden by the POH. More to the point, automotive oil is bad for aviation engines.

Aviation and automotive engines work on the same basic principles, but many of the details are different.1 One significant difference is that aviation engines normally burn some oil as they run, while well-performing automobile engines burn little or none.

Automotive oil contains a number of additives, such as detergents and wear inhibitors, intended for use where the oil doesn't burn off. Some of those additives do not burn completely, but instead stick around in the form of ash, where they can foul the engine and create deposits that lead to pre-ignition. That is why aviation oil is ashless dispersant: it cleans the engine by dispersing combustion byproducts into the oil (to be removed on the next oil change), while being ashless when burned. Lycoming considers it serious enough to put in an all-caps "CAUTION" section in their Lubricating Oil Recommendations document:

UNDER NO CIRCUMSTANCES SHOULD AUTOMOTIVE OIL BE USED. THE USE OF AUTOMOTIVE LUBRICANTS IN TEXTRON LYCOMING ENGINES IS NOT RECOMMENDED BECAUSE ITS USE COULD CAUSE ENGINE FAILURE.

If all of that wasn't enough, use of an improper oil may void your engine warranty. In short: don't use it.

References:

1) Note that I'm referring to air-cooled aviation engines that burn 100LL - made by, say, Lycoming or Continental. Rotaxes and new aero-diesels are different, and more like their respective automotive counterparts; in fact, Thielert engines are actually based on automotive diesels.

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Piston engines will, in general, accept any medium-weight motor oil without complaining. While you should, also in general, use what the manufacturer specifies you can get away with quite a lot depending on the amount and your intentions. One litre low, moderate climate, short trip, planning on an oil change anyway? 7-11 brand 10W30 will be fine. Ferrying across Alaska in winter with 2 "overnight" stops? Better use the good stuff.

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    $\begingroup$ Automotive oil can do some serious damage to an aircraft engine: The metallic detergents can form deposits in the cylinders that lead to preignition and potentially damage the engine. This risk would logically increase in proportion to the amount of automotive oil used. It's better to carry a quart of oil with you in the plane in case you're "a little low" than to top off with unapproved oil and risk engine damage. Using (synthetic) automotive oil exclusively has other issues as well... $\endgroup$ – voretaq7 Oct 13 '14 at 21:16
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    $\begingroup$ I agree, but that's where the "amount and future intentions" kick in. If you are 90 minutes airtime from home, are planning on changing the oil on Tuesday, and the engine is 50 hours from replacement the ash and deposits become a non-issue. $\endgroup$ – paul Oct 21 '14 at 14:16
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I used to read World-WarII adventure stories, one in particular rings in my mind.

A PBY landed at a remote location, in the South Pacific with one engine badly leaking oil. At the remote base all they had had on hand was cooking oil.

The pilot ordered the engine to be filled with cooking oil, intending to get the plane off the ground, then shut it down.

But the engine was running so well, that he kept it running and running, and running, and some hours later he landed at the home base.

The crew removed the engine, then sent it to the manufacture, telling the story, at the factory, they took the specs of the engine, and found that the cooking oil had done no damage.

The engine was fine.

. Moral of the story, you can run cooking oil in an aircraft engine.

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    $\begingroup$ Welcome to Aviation.SE. Please, we are looking for factual answers, if you have a source for your statements, please edit it in. As of now your answer is sounding like a mix of hearsay and urban legend. $\endgroup$ – Federico Oct 4 '17 at 20:26

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