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I'm planning to get a new petrol (gasoline) car soon.

A friend of mine, who worked as a motorbike mechanic when he was young, told me he puts a small quantity (125 ml / 4 oz) of 2 stroke oil in the petrol tank the first time he fills a car.

He argues it helps the burn (break) in of the engine, making the piston ring seal better against the sleeve, or somehow doing tolerance "matter less". I've found information in the web on doing so to diesel engines, but not to petrol ones.

Do you find doing so advisable? Anything to worry about if I do so? Does it sound logical?

There is a related question on breaking in new cars, but not covering this idea.

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My response to claims like this is "show me the data." The burden of proof is on the person making the claim, not the person saying "that sounds crazy." See also en.wikipedia.org/wiki/List_of_fallacies and lmgtfy.com/?q=butt+dyno –  Bob Cross Nov 29 '12 at 15:39
all the break-in involves is sealing the rings. That being said, many engines arrive today already broken in. If anything the 2-stroke oil could clog your injectors and cause unnecessary carbon build up in your engine. –  Chris Jan 23 '13 at 4:58
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up vote 5 down vote accepted

I agree with Bob's comment 100%

Do you want to risk your engine trying something unsupported by guidance from the manufacturer? This will invalidate any warranty from them!

Generally manufacturers give good guidance on running in engines (where necessary) including which oils, what speeds/revs you should use etc.

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