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When I change my oil I don't like going all the way back to the station to drop off the old oil and I don't want to pour it in the river, so I put 17 gals of premium in my 18 gal tank and then top off with my old oil. It's a regular gas car, not diesel. It runs a bit rough and I can barely see out the rear and it drives like it's underwater but a couple tanks later it runs like a champ. When I told my my friend who wrenches for a hobby he nearly blew a gasket, told me I'm lucky my car still runs and I can burn out my cat. I've been doing it for 7 years and haven't noticed problem. So, is this a bad idea or something?

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    A good way to quickly destroy all your catalytic converters. – Moab Sep 22 at 16:30
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    I don't understand how you reconcile the statements "it runs a bit rough and I can barely see out the rear and it drives like it's underwater" and "haven't noticed a problem." Because all of the symptoms you listed describe a serious problem – alroc Sep 22 at 17:18
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    If you're not going to take it to a shop, and for the love of god don't pour it in a river, but I'd have to think just putting in the garbage would be better than burning it. – Mazura Sep 22 at 20:43
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    "When I told my my friend who wrenches for a hobby he nearly blew a gasket [...]". I'm not a mechanic, but that reaction seems appropriate for what you've done. I'm sure that the manual of your car says to only put fuel in the ... fuel tank. – Ismael Miguel Sep 23 at 11:58
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    I'm just surprised anyone can reconcile "pouring it into the river is bad for the environment" with "making everyone breathe it is probably fine" – xyious Sep 23 at 21:02

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It's a BAD idea! The oil is contaminated with impurities that will not do the fueling system on your car any favors. Further, it's bad for the environment because your car is NOT designed to burn oil even if it's mixed with gasoline.

The best thing to do is to take the used oil to a place where it can be recycled into new oil and used again. Most oil change places accept it, many communities have recycling drop off locations, many auto parts stores take used motor oil.

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    The questioner could get busted by the traffic police where I live for emitting excessive smoke, and made to get the car towed. – Michael Harvey Sep 22 at 15:29
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    I completely agree on this one @Rob, this is actually high on the list of terrible ideas. Please stop for the good of your car, if not other reasons. – GdD Sep 22 at 15:51
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    They're not exactly comparable, but what's the carbon footprint of a new engine compared to a few quarts of oil added to the waste steam.... – Mazura Sep 22 at 23:53
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    @Mazura The oil added to the waste stream would eventually decompose, so the carbon footprint is the same for either approach. Of course co2 is not the worst pollutant you add to the athmosphere in this case. – Taemyr Sep 23 at 9:31
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    @Mazura In this case, wouldn't the carbon footprint include both, since you're burning oil and working your way towards a new engine? – JMac Sep 23 at 20:00
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Aside from the numerous reasons not to do this to your car I would like to offer you a solution to your perceived problem.

Don't take the old oil back to the store. At least not right away...

Your new oil was purchased in single quarts or a 5-quart jug, right? Put your old oil in the empty containers and recycle it at your convenience. This could be the next time you go to the store or the next time you change your oil.

If you don't have new oil containers for whatever reason then how about a milk jug or get a 5-gallon bucket with a lid.


As a side-note, what car do you own exactly? It sounds like it would survive the apocalypse and run on anything you put in the tank so I may wish to buy one.

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All modern cars scavenge blow-by including oil vapour and a mist of droplets. This is good for upper cylinder lubrication and does extend ring and bore life.

If you have to, you can install a catch can to condense steam and larger sludgy solids. Older cars just vented through a wire wool cap on a rocker cover.

Used oil is another matter.

There is going to be metal in that oil, some of it will be larger than 15 microns, it has come from the sump after all. You need to filter it some more before it goes any where near precision parts such as injectors. It is probably fine for pliers, seized agricultural nuts and the like. It will not be good for your skin or for food. unleaded petrol is not green, it is evil stuff, and that oil is full of nasty contaminants.

In your petrol or gas :

  1. It will foul spark plugs. 2 stroke oil and 2 stroke plugs are intended to function as expected when used properly.
  2. It will shorten the life of your oxygen sensors.
  3. It will shorten the life of your catalytic converters if still fitted.
  4. It will add deposits in your other mufflers until they weigh too much and drop off, or start causing back pressure issues, over and above catalytic converter issues.

It will save you fuel, if it is not too much trouble to rebuild, you could put it into a B & S, but you might have to re-jet.

I suggest adding it to charcoal or smudge pots in winter, it is fuel. Just don't breath the vapour. I know backyard blacksmiths that might use it for quenching something from the forge, but they don't want to breathe it either.

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    If you have removed your cats, how do you keep your oxygen sensors from blowing codes? OBDII cars have O2 sensors on both sides of the cat, they'll notice. – Harper Sep 24 at 22:16
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The main reason for changing synthetic oil rather than topping it off is to get rid of eroded material. Pouring it back into the gasoline side would not even be a good idea if you were driving a diesel car. Putting it into a gasoline car is pure madness that would smoke your cat pretty fast even assuming that the oil is fresh and free from any abrasions: there is a reason this is a gasoline car and not a diesel.

But even diesel cars these days are quite specialized and have high compression rates incompatible with significant amount of messing with the fuel. A tractor from the 60s will be comparatively tolerant to tampering with fuel like that (but you still wouldn't want to fill in metal filings) but comparatively modern cars should get the fuel they are certified for and nothing else. They get their mileage by relying on the fuel's behavior before and after ignition under very high compression ratios.

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It really does hurt your car. Just think about it, your car isn't made to process this type of liquid, so why would you just dup it in? Its basically like dumping tar in your tank and expecting it to run. Yeah, not exactly good for any of your car parts. Your friend is definitely right that you are lucky you haven't had any real problems! You mentioned that you do this to not "pour it in a river" which I assume means you are trying to be a good citizen and not destroy our water supplies! But, air pollution can be just as bad to worse. That air is the thing you will die the fastest without. So, pumping this impure old oil into the air is probably a horrible idea. Now I'm definitely not saying you should go and dump ur old oil inside of a river that is also a horrible idea :v (I mean come on). But what you really need to do is just take it to a gas station, it will end up saving the environment, a lot of money, and a lot of extra time from getting ur car fixed constantly! For the love of Pete take it to a gas station. Hope this helps from Locomotive 38!

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$5-10 extra for a premium tankful makes this a rather expensive way to dispose of old oil. To say nothing of engine parts such as the cat and O2 sensors. And fuel filters, cringe.

If this was literally my only way to do that, I would a) let it sit for a month to settle out metallics and soot, and b) filter it to 5 microns.

Then add it at less than a quart per tankful, reducing further as needed so the engine doesn't smoke. Smoke means incompelete combustion means you are torturing the catalytic converter to death (never mind the metallic junk in the oil if you don't filter it, which will poison the cat). And running the fuel injection systeem out-of-gamut: meaning the fuel injection system is designed to adjust for variations in fuel and still run clean and stochiometric, but this fuel is too different from normal fuel and is beyond the system's ability to adjust for it. Reduce the ratio.

Really, a better plan is to do the settling and filtration I mentioned, then blend it into an old diesel engine at 50:1 or so. I am unclear how you can be so far from civilization that it's a chore to take used motor oil to an auto parts, and yet, there are no farmers around?

I would not use it for 2-cycle oil as that is too critical an application.

Or even better, after settling, use it as makeup motor oil to replace oil burnt between oil changes. Then you don't need to do the filtration because filtration needs for oil are different than for fuel... Also the car has an oil filter. I hope you've change it from time to time.

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It should be noted (since nobody else did) that your car isn't designed to burn an oil-mix and that cloud of smoke behind your car is polluting the neighborhood. I am surprised you haven't managed to foul your plugs doing this.

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Interesting approach. Caterpillar issued a TSB probably 10 years ago, which prescribed dilution rates of crankcase oil to fuel for their off road equipment like bulldozers. Of course those are diesel, and they may not have a catalytic converter to deal with.

In a gas car, I would likely avoid the issue. However, I will add that a tribologist friend pointed out to me several years ago that gasoline can be up to 5% oils of various sorts. But if you are leaving a smoke screen behind you, you are most likely coating your catalytic converter with stuff that someday might become very hard to clean off.

The 11 micron oil filter will keep particulates to a manageable sizes, something the Cat bulletin pointed out. However, I can't help think that there might be some stuff settled into the bottom of the sump that your sensitive Swiss-watch manufacturer built fuel injection system might gag

So being a bit conservative, your scheme might work, but I would spread the load out over the fill-ups until the next oil change. That should keep your total oil percentage lower, and help those who follow you from having to use Dawn to suds down their windshield.

Afterthought: We don't know the OP vehicle, and more specifically whether it has a catalytic converter. We also tend to think rather narrowly in what is a "good practice." For example, biodiesel is an effective fuel system cleaning agent. (A reason to avoid it in some cars.) I had a teenage daughter who traveling with her friends, was low on fuel, and used vegetable oil from a grocery, priced less than diesel fuel, to get home. There was no apparent harm to the diesel engine in the car. However, stories in novels and non-fiction works have addressed the historical use of "alternative fuels."

  • For those who unfortunately don't have a tribologist friend, that is of course someone who studies friction, wear, lubrication, and the design of bearings; the science of interacting surfaces in relative motion. – Glen Yates Sep 24 at 19:27
  • Yes, and they are smart people about stuff that has lots of folklore behind it. – mongo Sep 24 at 23:48
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Any 'quick lube' joint or auto parts store can accept used oil. No need to 'go all the way back', whatever that means.

  • In the US, anyway. – mike65535 Sep 24 at 12:51
  • @mike65535 which is where OP is. He said so flat-out, "pour it in the river" ... – Harper Sep 24 at 22:50
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Hope this is a joke!

Since you happen to live in an area where a heavy smoking car and/or non-operational catalytic converter does not attract the due regulatory attention, you can as well continue to do so. You will actually save if you don't use premium gas in the mixture.

Resisting to throw the oil in the river is good, but throwing it in the atmosphere is no better or worse. It can be even worse as the sewage is usually treated in one way or another before reaching the river.

If you happen to have a friends with older (non-common-rail) diesel cars/trucks/busses, they can burn the waste oil for you. The environmental harm will be no worse than burning the equal ammount of diesel fuel.


Seldom used older cars w/ 4-stroke gasoline engines actually benefit from some (2% or less) 2-stroke oil pre-mixed in gas, but the usual (4-stroke) motor oil in high proportion will surelly do more harm than good.

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    Every engine that would benefit from a slight addition of oil would expect new oil, not used oil that's full of tiny metal shavings and has become abrasive. – Peteris Sep 24 at 11:17
  • Fair point, but the engine was lubricated by the same oil with the same ammount of wear products just few days ago, so I don't expect some drastic abrasion to happen from one more pass. – fraxinus Sep 25 at 10:07

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