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Here's a practical chemistry question for you. Do you know what happens if you mix regular and synthetic motor oil? For example, let's say the mechanic put synthetic oil in your car when you got your oil changed. You stop at a gas station and see you are running about a quart low, but all you can get is regular motor oil. Is it better to use the regular oil or will you risk harming your engine if you add the oil?

According to Mobil Oil, it should be fine to mix oils. This manufacturer states it would be unlikely anything bad would happen, such as a gel forming from an interaction of the chemicals (a common fear), because the oils are compatible with each other. In fact, many oils are a blend of natural and synthetic oils. So, if you are low on oil, don't be afraid to add a quart or two of synthetic oil if you are using regular oil or even regular oil if you are using a synthetic. You don't need to rush right out and get an oil change so you'll have "pure" oil.

However, it is not recommended to routinely mix oils because the additives in different products may interact or the oils may become destabilized by the mixture. You may reduce or negate the properties of the additives. You'll lose the benefits of the more expensive synthetic oil. So, adding regular oil to your special synthetic oil will mean you'll need to get your oil changed sooner than you would have otherwise. If you have a high performance engine, it's possible it will be displeased if the (expensive) additives can't work the way they are supposed to. This may not damage your engine, but it won't help its performance.

Have you ever mixed oil, either accidentally or on purpose? Was your vehicle fine or did it suffer any ill effects? You're welcome to post a response.


January 31, 2012 at 4:26 pm
(1) Rick says:

I heard that synthetic ‘blends’ are made from ‘recycled’ oil with some synthetic added. Is this true?

February 6, 2012 at 6:40 am
(2) zvi erez says:

the blends are called “semisynthetic oil”. it’s annoying that they don’t state what percentage of synthetic oil they contain.

February 6, 2012 at 9:53 am
(3) Frances says:

Has anyone ever switched a vehicle from “regular” to “synthetic” and noticed a difference like change in idle speed (when you didn’t adjust the speed)?

February 6, 2012 at 12:14 pm
(4) Chemist says:

Synthetic Oils are known to have VI Viscosity Index greater than 100 (even before adding additives), designed to extend the use of the oil for much longer distances relative to petroleum-derived oils (VI less than 100 before using additives). Consequently, I don’t think of a probability for blending recycled oils (expected as petroleum-derived oils) to synthetic oils.

July 30, 2012 at 7:28 pm
(5) donn says:

Ok i bought a used car today lets say the previous owner used syntheci oil and the dealer ship put regular oil in it not knowing it had syntheic oil,would this change caus the motor to knock??

November 4, 2012 at 9:39 pm
(6) sfla says:

Switching from synthetic to conventional will not cause engine knocking. If the sound is more of a chattering, trying running higher octane fuel as worn engines sometimes have higher compression than when they were new. This causes the compression of the piston to ignite the fuel before the piston reaches its zenith and before the spark plug fires. Higher octane fuel is harder to ignite with compression and will allow the spark plug to ignite the fuel. If the sound is definately a knocking, the motor is on its way out. Sometimes, people will run a heavier weight oil to stop the knocking long enough to sell the vehicle.

November 20, 2012 at 6:25 am
(7) Hazem says:

I mixed mineral with synthetic and it works fine….

January 21, 2013 at 9:21 am
(8) luke says:

I have never had any issues with it. My car takes 4quarts, so I use 3/4 semisynthetic and 1/4 full synth, its just a matter of cost. It is much better than conventional. I recommend using the same brand so that they contain compatible additives. I can’t imagine using different oils could actually cause damage.or even noticeably affect performance. And if you are swithing between full conventional to full synth, just drain it, replace filter, add new. No problems.

February 13, 2013 at 12:20 pm
(9) gane says:

Synthetic oils means made up of PAG / PAO /Esters chemicals similar to those of mineral oils. It is not made of any recycled oils. They have very high VI(Viscosity Index) more than 140 plus, very High oxidation stability, Shear Stability & thermal stability, low pour point, When compared to mineral oils they have longer service life in addition to their extended overhaul periods. Synthetics are clearly superior in the extreme zone where temperatures, high loads or flammability are overriding factors. They also perform well in applications where needs are specific and complex. Synthetics are engineered to meet targeted performance benchmarks, and a synthetic formula can be (and probably has been) engineered for almost every combination of properties used in industry.

July 5, 2013 at 11:44 am
(10) Pete says:

I’ve mixed different types of fully together, different types of semi and on occasion, fully with semi. Never had any negative results.
Go to 2 different garages and they’ll tell you 2 different things about which oil is best for a 100k miles plus motor etc. They all tell you like it’s gospel.

July 17, 2013 at 10:07 am
(11) Etienne Burger says:

I do;nt think synthetic oil should be mixed with natural oils, because the viscosity of the end product will be affected which is bad for your engin and the beneficial adatives will be affected negatively.These new oils aren’t synthetic or artificial in the sense that they’re manufactured out of whole cloth–they still have the same natural ingredients found in “real” oil. But in a synthetic lubricant, these ingredients are recombined like a Lego set to yield synthesized-hydrocarbon molecular chains with desirable characteristics and uniformity not found in even the highest-quality traditional motor oils. Typically, the best synthetic oils use a combination of up to three different synthetic base fluids–polyalphaolefin (PAO), synthetic esters, and alkylated aromatics.

Because a synthetic oil’s molecules are much more consistent in size and shape, they are better able to withstand extreme engine temperatures. By contrast, the unstable molecules in conventional oil can easily vaporize or oxidize in extreme heat. Mobil 1 synthetic is said to be capable of protecting engines “at well over 400 degrees F”; in the real world, most racers have no problem running synthetics up to 290 degrees F under prolonged use, but they get really jumpy when a conventional exceeds 270 degrees F.

July 17, 2013 at 10:20 am
(12) Etienne Burger says:

Synthetic oils, pioneered in the ’70s by Mobil and now available from most major oil companies, take the all-season, multiviscosity approach to the outer limits. Unlike traditional mineral oils that are produced by distillation and further refining of existing crude oil stock, synthetic lubricants are made through chemical reactions. These new oils aren’t synthetic or artificial in the sense that they’re manufactured out of whole cloth–they still have the same natural ingredients found in “real” oil. But in a synthetic lubricant, these ingredients are recombined like a Lego set to yield synthesized-hydrocarbon molecular chains with desirable characteristics and uniformity not found in even the highest-quality traditional motor oils. Typically, the best synthetic oils use a combination of up to three different synthetic base fluids–polyalphaolefin (PAO), synthetic esters, and alkylated aromatics.

Because a synthetic oil’s molecules are much more consistent in size and shape, they are better able to withstand extreme engine temperatures. By contrast, the unstable molecules in conventional oil can easily vaporize or oxidize in extreme heat. Mobil 1 synthetic is said to be capable of protecting engines “at well over 400 degrees F”; in the real world, most racers have no problem running synthetics up to 290 degrees F under prolonged use, but they get really jumpy when a conventional exceeds 270 degrees F.

July 22, 2013 at 4:32 am
(13) PooPooPeeDoo says:

If you mix half normal oil and half fully synthetic oil 2 bad things happen. The normal oil becomes twice as slippery and the synthetic oil becomes half as slippery.

So to use the diametricol constants of the molar hysterisis, that 2 x 1/2 = 1; so the lubricity becomes 1 times the mixture of the both.

When your engine is running at idle – it is the state of 1 – say 1000 RPM. When your engine is spinning at 4000 RPM, the oils capacity to lubricate is one, but it is out driven by a factor of four.

An engine being lubricated by a factor of one quarter, will suffer catastrophic damage.

Thus your engine will be rooned.

Do not mix natural juices of dead dinosaurs and synthetic juices of non existent dead dinosaurs.

July 22, 2013 at 6:00 am
(14) Steve says:

I’ve spent many years with major oil company’s. The best way to simplify the concern of mixing motor oils is to make sure you are not mixing Polyalkylglygol (PAG) based oils with any other oils. Mineral, polyol esters (POE) and Polyalfaolifin (PAO) based oils are O,K to be mixed. However some of the benefits may be lost in mixing.


July 22, 2013 at 8:47 am
(15) David Salmon says:

Use the oil as specified by the engine manufacturer. BTW, I think “AMSoil” was the first to make synthetic oil commercially available (not Mobile).

July 23, 2013 at 1:11 pm
(16) Carl Piepora says:

Been using 5w30 Mobil 1 synthetic oil in a GMC 5.3 vortec engine for 350,000 miles. Recently I had an engine oil level sensor replaced and needed to top off with 3 qts of regular oil. No problems, but my next oil change will be back to synthetic.

July 27, 2013 at 4:15 pm
(17) roy says:

I just change my oil mix 1qt with regular high mileage check it every thing look good, about hour later check it again car look calkish in color, not running properly like a skippin in the engine. What is wrong?? Help.

August 14, 2013 at 3:52 am
(18) synergyge says:

Synthetic Oil is the best part of oil because engine life increses alot due to less evaporation loss.it also help to improves fuel efficiency,Better lubrication on cold starts,Increased resistance to oxidation, thermal breakdown and oil sludge

August 28, 2013 at 5:11 pm
(19) CrustyBiker says:

I mixed oils for my motorcycle engine/gearbox while I had left over oils of different brands and I was waiting for a new tub of fresh good quality oil to be delivered, when it came I still carried on with the mix until I found a convenient time to do the change, and one day my gearbox felt noticeably stiff changing and would often miss gears, I’ve had to purchase engine flush, and I hope to the bike gods it works as I will be extremely p155ed off if I’ve knackered my synchromesh just because of impatience, laziness or saving monies. I will let you know if problems are permanent after flush and replenish, fingers crossed!

August 31, 2013 at 11:35 am
(20) Gumby says:

Just keep an extra quart of same oil you use in your engine in your trunk.

September 16, 2013 at 8:53 am
(21) Dusan Ilic says:

Is there any real reason why a synthetic motor oil would not be used in cars that have higher mileage, or in cars that are produced in the 70′s or 80′s? Volvo dealer does not advise use of synthetic oil even on a b230 engine with (factory) turbocharger because “it is an old technology when they did not put synthetic oils, and oil consumption or oil leaks would be higher???!!!” If an oil has the same viscosity, how can such difference occur?

September 21, 2013 at 10:02 am
(22) No it all says:

Older engine seals typically harden and synthetic oil slips through the porous hardened rubber more easily due to the smaller molecules then dinosaurs cause they’re big (molecularly). A turbocharged older engine shouldn’t experience it as badly, as they typically run at lower compression then Naturally aspirated engines. This seepage occurs most often with older motorcycles as they run at much higher RPM’s and compression. Viscosity is not actually a measure of the size of molecules, but rather a reaction to heat. Also, M and M’s sometimes do melt in your hand.

October 17, 2013 at 2:56 pm
(23) Darrell says:

Just lost my engine because I mixed synthetic with an old quart of HD 30 Pennzoil I had in my garage cabinet left over from way back when. Not sure what happened, but it only took 20 miles and all HELL broke loose in that engine, a 5.3 litre GMC Yukon engine that was running great before that. I had 111,000 miles on engine and had always changed oil on time, or usually way before time. There was a chemical reaction that only a chemist would probably understand, but it was bad. All of the hydralic lifters started clattering and bascally quit functioning and opening the valves. Oil pressure was eratic, bouncing around between 10-30 pounds, normally 40-50 pounds, and and check engine light came on. We got it in to a shop and they drained oil, sludge, sticky sludge out, and tried flushing with a strong flush without results. Two oil changes and flushes and could not get lifters working again. Engine basically trashed! $7000. mistake on my part. Will never mix oils again.

November 12, 2013 at 6:48 pm
(24) Tracie says:

Darrell: Your problem was mixing SAE30 with a synthetic that was probably listed as a 5w-30 or 10w-30 SL product. SAE30 would surely NOT be comptible mixture! It is for very old trucks, suvs, etc. not for today’s conventional engines. It’s fairly sad this day in age with the internet that a few seconds googling could have saved you several oil changes and a 7000.00 in damaged engine. Hindsight is 20/20, isn;t it?

November 19, 2013 at 7:46 am
(25) carignorant says:

I have an 11 year old Clio and my oil light came on. I had used a fully synthetic oil but when I went to buy some oil noticed that mineral oil was recommended for older cars so I bought that. However, I haven’t used it yet as am worried about mixing the oils. Any thoughts please? Thanks.

December 2, 2013 at 10:16 am
(26) bob says:

to SFLA: worn old engines have “sometimes higher compression” – REALLY, were did you get that from, can you share a source!

December 9, 2013 at 3:44 pm
(27) Melvin Burns says:

I mix oils all the time. Sometimes I use conventional. Sometimes I use synthetic. One time I ran out of motor oil so I just started taking all the kitchen oils in my house and pouring those in (vegetable oil, corn oil, olive oil, stir fry oil, Emeril’s Bam oil, and a few others). The car drove for a while then it started smelling like Emeril was under my hood yelling “Bam Bam Bam” while cooking something insane. Then the hood started smoking like Emeril was failing at cooking something insane. That’s when I stopped the car and just did a full oil change.

Morale of the story… don’t use non-motor oil in a motor unless you hate your car.

December 9, 2013 at 3:56 pm
(28) Alvin says:

Yeah, you can mix them. The guy at the car shop told me it was fine.

January 6, 2014 at 10:44 am
(29) crownvic says:

I recently was gifted a free oil change from a service manager from a VW dealership but i have been using full sythetic oil for seven straight years on my 98 crown vic and im pretty sure if I redeem my free oil change they are going to use conventional oil should i risk it?

January 6, 2014 at 4:52 pm
(30) chemistry says:

I usually use full synthetic in my Corvette, but there have been a few times when I have gone with regular oil. The mechanic said it was fine, the car seemed happy, and the only downside was I couldn’t go as long until the next oil change. If your car has specific requirements, they will put additives in whether you use regular or synthetic oil so it isn’t all that different in the end.

January 18, 2014 at 1:24 pm
(31) tteksystems says:

All the different comments you might read, good or bad, regarding the mixing of oils is mostly to control your thinking and will not likely guide you to a more cost saving effort to use the best oil type or brand of oil. Unless it’s coming from an unbiased EXPERT.
People who are very particular about using the best oil while saving money will often mix a couple quarts of synthetic with non-synthetic oil at every oil change. Synthetic is expensive. And to say it might interact and rob you of the true benefits of a product, when mixing with another, will leave me with the same impression people say ” Once you switch to synthetic you have to keep using it forever”
This article was good and gave good information and should take the fear from those who did not know if it is OK to throw a quart of synthetic in your car when it was the only option. This article also mentioned it might not be a good idea to regularly mix synthetic with regular oil as it might rob you of the true benefits either of the oils are promoting. That is a rational assumption. But I think all the important benefits for protecting your engine are properties of every oil and I highly doubt there is a problem using 1 or 2 quarts of synthetic with non-synthetic in order to save a few dollars on oil changes.
One rule of thumb someone told me is that synthetic is never a bad choice. To use synthetic in a car that specifies regular motor oil is not a bad thing. But to use regular motor oil in a car that specifies synthetic motor oil is never good. So I wouldn’t mix oils in a vehicle designed to use synthetic

January 25, 2014 at 11:06 am
(32) dave smith bristol UK says:

Looks like Mineral and Synthetic are ok but I recall being told back in the 70s when motor racing used something called ‘Castrol R’ that it would not mix with mineral oil, it being a ‘vegetable oil’. The engine ‘expert’ said that mixing vegetable oil and mineral would make the engine ‘gum-up’.

Getting the right viscosity (for hot or cold conditions) is another issue.

February 10, 2014 at 10:01 am
(33) MindControl says:

Yes, I am trying to control your thinking…..send me all of your money.

I dont care what u think, added 1 qt to synthetic blend and lost all oil pressure in 2 weeks.

real world experience, take it for what u want to.

February 10, 2014 at 4:38 pm
(34) Mark J. says:

To Bob and others wondering about older cars possibly having higher compression… in the older days it was well known that a high mileage engine could have higher OCTANE requirements. I believe the reason was buildup of deposits in the combustion chamber.

Of course we also know rings wear and leak more, so it is hard to be sure which factor will dominate.

March 17, 2014 at 5:11 pm
(35) Lanix says:

Not a comment but rather a question.
Can mixing brands of oil together in engine cause it to start smoking and severe shortening of oil?
please enlighten me, that is what I experienced .

March 26, 2014 at 12:33 pm
(36) Tony says:

Do NOT blend it !
I blend different brand Synthetic and Conventional about 2:1. The gas consumption goes higher from dashboard computer reading for both highway and local. I changed back to conventional after about 100 km. the oil drained out is very thin like water – some reaction may occur.

March 27, 2014 at 11:06 am
(37) Littletech says:

There are some very opinionated people here… I find it hard to believe that some people get so finicky about their oil… They must all have race cars or something… Oil is oil just make sure your using the same brand. It’s comical that people are saying don’t mix the two because obviously they are smarter than these multi million dollar oil company’s with labs the size of a city block doing research and saying synthetic blend oil is safe for use in your vehicle and then selling to dealerships who the put it in your car… If your car breaks after you added conventional oil to your synthetic oil there was something wrong with your car to begin with or you don’t know how to read a label. Remember the three b’s! didn’t build it break it or buy it and everytime you turn that key something is wearing out. I rest my case

April 2, 2014 at 6:17 pm
(38) 2001 Grand Vitara says:

I believe there is another factor involved in the oil types. I have been using Castrol Syntec in my Suzuki Grand Vitara V6 for 70,000 miles. Never a drop of smoke.

Had an oil change recently and conventional oil was put in instead of Synthetic. The vehicle smoked like a Mosquito Smoke Buggy for 10 miles!!!

The next day I figured out what happened. I drained the oil and put Synthetic back in. After only 10 miles the new conventional oil was very dirty. This is due to the High Detergent content of synthetic oils.

Pick one type of oil or the other but DO NOT switch back and forth!

April 9, 2014 at 4:27 pm
(39) madmantrapper says:

I think there are a few on here smoking something.

April 29, 2014 at 9:29 am
(40) John Mack says:

There is no problems mixing synthetic with natural oil. You’ll see the same manufacturer like Shell have a full synthetic and a part synthetic/blend. The latter is a mixture of 3 qts synthetic and 2 qts natural, Same oil filter is used for both, I have a new car and use full synthetic but on my old car with 125K I use high mileage oil – the slick synthetic would destroy the car.

May 7, 2014 at 10:16 pm
(41) Master Tech says:

To ALL OFF YOU Dip-Sticks out there that THINK you know Oil’s & Engine’s.. I AM a A.S.E. Certified Master Auto Tech w/ Over 30 Yr’s of Experience & I can tell you ALL that YES it IS FINE to MIX Full Synthetic, Semi (blended) Synthetic & Conventional Oil’s in your Engine. IT WILL NOT harm your Engine at all to do so. As for the guy that spent $7000. on a new Engine (cuz a shop said your Old one was toasted from using the WRONG OIL), You, my friend, Got shafted. What happens in SOME Engines, (such as yours), is that using Synthetic Oil brakes down & flushes hard deposits threw the Oil Passages & they become trapped inside the Lifters. Causing them to become plugged & stop functioning. This could have been repaired by removing & cleaning or replacing the lifters. NOT A NEW ENGINE…. And to the Jack-#ss that says Not to use Synthetic in Older Engines, You Clearly DO NOT KNOW WHAT YOU ARE TALKING ABOUT AT ALL. Because Synthetic Oils where Designed specifically for Older, Higher Mileage Engines… And, To the Idiot that claims that Old Higher Mileage Engines Have Higher Compression, You Truly Are an IDIOT…. The Truth Is That Older Engines have More ware on the Cylinder Walls & Piston Rings, Witch Causes said Engines to have considerably LESS compression than a newer low mileage Engine. I HAVE Built & Rebuilt Manny, Manny Engines in my time as a Tech (from little compact car Engines to Massive Big Block v8 Drag Race Engines Pushing Well Over 3000 H.P. & I have NEVER Had Internal Damage occur do to Mixing Oils.. So, To ALL OF YOU OUT THERE, IF YOU DON’T KNOW WHAT YOU’RE TALKING ABOUT….DON’T ACT AS IF YOU DO !!!!….

May 8, 2014 at 6:04 pm
(42) _____ __ __________ says:

To keep this piece brief I’ll focus on full synthetic oil, API Group V, as opposed to semi
synthetic oil. There is also another body, the International Lubricant Standardization
and Approval Committee (ILSAC), jointly set-up by US and Japanese motor
manufacturers, which also recommends motor oil grades which comply
with the API standard. According to the federal government’s Magnusson-Moss Act a manufacturer can’t tell you
what specific product you must use to save your
warranty they can only tell you what standards that the product you do use has to live up to.

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