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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 and – 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
This is only really a thing with rotary engines (which act more like 2-stroke engines than 4-strokes). – Juann Strauss Aug 17 '15 at 8:03
By the time you drive off in your new car the engine has been run a multiple times. Putting the oil in the fuel is a waste of time. He would have to add more than 4oz of oil in order to make a difference. It gets diluted into nothing. Lets say he did add more. You know what would happen? The car would run badly or not start. Look at it from this point of view: Does your friend who is a motorbike mechanic know more than thousands of engineers that design these cars? – race fever Dec 23 '15 at 15:23
up vote 7 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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Have you ever opened up an engine that is using oil, in other words, oil blow-by past the rings. The piston with the oil consumption is clean where as those burning correctly have a layer of carbon on top of the piston. Now that is ordinary engine oil that has removed the carbon. Therefore I fail to see how a very diluted mixture of two stroke oil will damage the engine.

And for those people that say yea but the fuel companies would have added it to the petrol.....

Look on at the demo on you- tube at omega or lubrication engineer products. and see how they exceed your standard lubricants sold by the petrol industries. If the petrol industries were so keen to give you the best product then they would match the above lubricants. Guess what, Ive seen the demos done personally and have used both products above in vehicles and industry. The above mentioned lubricants reduced machine failures tremendously in the industry that I worked for. Infact on one product in particular eliminated regular hydraulic pump failure completely. These pumps used to fail once a month on hydraulic oils supplied by the petrol industry. And we tested all brands. And no, I am not advertising, thats why I did not mention which one of the two products it was.

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Imho adding 2 stroke oil will help lubricates the top part of the piston. What i am worried is, how clean the mixture will burn? Because most 4 stroke engine have egr. If it burns much dirtier, u could end up with more soot re-entering your combustion chamber.

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I was mixing motor oil in the fuel tank of my petrol car for a year and it is giving me better performance on hills as well as the engine is still young because i am driving it very roughly still it has a very good pick up and performs really good. Quantity of oil I mix is about a single pouch in 10 liters of fuel.

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Uh, really? I hope you like a plugged catalytic converter, because you are well on your way for that to be happening. – Pᴀᴜʟsᴛᴇʀ2 Dec 23 '15 at 15:28

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