Explain why it is bad to mix 2 different oils?

[lipadj46]

I hear people say you should not mix unlike oils and they give 2 different reasons. Reason #1 is that some oils do not mix. So question 1 is that is polarity of some oils so different that they would not mix? I know I here people talk about ester oils and these would be polar enough to not mix with a non-polar oil?

Reason #2 is that certain additives in one oil would cancel out an additive in another oil. So can someone give me examples of these reactions? I'm a chemist and chemical engineer so you don't have to spoon feed it to me I'm just curious.

 

[bdcardinal]

because the thought of the Castrol "think with your dipstick" Jackie Stewart knock-off riding the Mobil pegasus makes me wake up in a cold sweat.

if it matters i just topped off my Volvo with Q 10w30 which has German Castrol in the sump and everything is ok. why Q you may ask, well i had a random bottle floating around the garage.

 

[lipadj46]

I'm just curious to know if there is an actual scientific reason. I hear people on other forums say it with quite a bit of authority but if pinned down for an exact reason they always say it is just what they have read and have no specific examples. I am hoping a tribologist or other chemist can explain one way or the other.

 

[Mustang_Cougar]

I've yet to read of one example on this website of the detrimental effects of mixing SL/SM oils. In theory, sure..."additive clash" is a possibility. But with all the UOA's and personal histories here, you'd think we would have witnessed one documented case of engine damage or failure based upon additive clash. Yet there are none, to my knowledge.

I'm beginning to think additive clash is like Bigfoot: A lot of people know of it, a lot of people belive in it, but nobody has real-world evidence of it. Until then, mix away...it's better than maintaining a collection of orphan quarts in your garage.

BTW: I know the search function isn't the best, but this subject has been thoroughly vetted before, numerous times. You might find better info/replies/science inre this issue with a search...

 

[John_K]

Someone on here once said their best UOA was with a mix...try search (or use Google, I find it searches this site better).

I think it's just a bunch of "might do this, might do that".

John

 

[lipadj46]

I am searching but like you said it is not the best. I'm sure it has been covered ad nauseum and I hope I'm not dredging up any old forum baggage with a new post. Any links would be great.

My thoughts are along with you guys that is mostly "soft" science. I hear people say additive clash and the only thing I can think of that would be terrible would be if 2 (or more) additives could react with each other and precipitate out or form a compound that had no desirable lubricating qualities.

 

[Boomer]

The only chemistry that pops into my head is that one oil might use a cationic add (quat salt)while another might use an anionic add (carboxylic acid salt). These could "salt out" to push both out of solution. But I am not aware of this situation in actual fact. Perhaps one of the oil formulators would care to chime in.

 

[Mustang_Cougar]

Originally Posted By: lipadj46
I am searching but like you said it is not the best.


Yep. Lately, I just go to Google and type what I'm looking for (ie, "5w30 Corolla") with the word "Bob" included; often times it gives me better results than the website search function.

 

[lipadj46]

I will try that thanks.

 

[d00df00d]

Originally Posted By: lipadj46
Reason #1 is that some oils do not mix. So question 1 is that is polarity of some oils so different that they would not mix? I know I here people talk about ester oils and these would be polar enough to not mix with a non-polar oil?

Hogwash.

I'm pretty sure it's been a long-standing requirement for any engine oil to be able to be mixed with any other.



Originally Posted By: lipadj46
Reason #2 is that certain additives in one oil would cancel out an additive in another oil. So can someone give me examples of these reactions? I'm a chemist and chemical engineer so you don't have to spoon feed it to me I'm just curious.

I don't think it's about "canceling out" as much as competing chemistires.

 

[demarpaint]

After spending time here, and doing a lot of reading, I actually think it is another myth. In the perfect world oil companies would like us to only use their oil and not mix it, or ever change brands. Then on the other hand they tell you it is compatible with other oils.

FWIW I've been mixing the orphan qts I have laying around and haven't destroyed my Aerostar in the last 2 OCI's.

 

[Geonerd]

Most oils use more than one anti-wear additive. These compounds chemically bind to the surface, creating a thin layer of sacrificial material. A given oil will have closely balanced chemistry that maximizes the effectiveness of each additive, and the additive suite as a whole. Mixing of oils may upset the balance, resulting in less-than-ideal interaction of the anti-wear compounds. For example, dumping a large dose of Zinc/Phos into an oil may tend to saturate the available metal surfaces with ZDPN, leaving the Moly, Boron, etc. anti-wear compounds fewer free locations to 'mate' to the bare metal. This is unlikely to be catastrophic, but some level of reduced performance seems possible.

(Where's Mr. MolaKule been hiding out lately? We need some sort of "Bat Signal" to summon him during these times of crisis!)

 

[lipadj46]

Originally Posted By: Geonerd
For example, dumping a large dose of Zinc/Phos into an oil may tend to saturate the available metal surfaces with ZDPN, leaving the Moly, Boron, etc. anti-wear compounds fewer free locations to 'mate' to the bare metal. This is unlikely to be catastrophic, but some level of reduced performance seems possible.


I could see that maybe. But in that specific case you would still be protected by the layer of zinc? I suppose there could be a minimum conc. for any additive to be effective and you could be say diluting a high ZDDP oil with a high HTHS oil (not sure if such a beast exists, still pretty new to oil) and diluting either component to the point where neither additive is effective.

 

[friendly_jacek]

Originally Posted By: Geonerd
For example, dumping a large dose of Zinc/Phos into an oil may tend to saturate the available metal surfaces with ZDPN, leaving the Moly, Boron, etc. anti-wear compounds fewer free locations to 'mate' to the bare metal. This is unlikely to be catastrophic, but some level of reduced performance seems possible.


Moly forms layers on TOP of complex organometalic compounds facilitated by ZDDP. Probably similar story with Boron.

The only competition is between surfactants and antiwear additives. However, most oils have similar ratios of those.

 

[Eddie]

Each oil is formulated with precise levels of additives. If you mix, you will not have the best formulation. probably no harm is done but, not the best thing to do.

 

[AEHaas]

Oils will mix almost uniformly without anything precipitating out. However some additives may be diluted. For example, most oils have ZDDP as one additive (it actually performs several different functions). A few have none at all. Instead, other combinations of different additives perform the multi duty ZDDP functions. If you mix half of each type of oil together then each ones special formula concentration is cut in half and may not work at all.

This is but one example of a very complex formulation that is typical of motor oil. Few actually know the whole formula even within oil companies, they are guarded secrets.

aehaas

 

[demarpaint]

Originally Posted By: AEHaas
Oils will mix almost uniformly without anything precipitating out. However some additives may be diluted. For example, most oils have ZDDP as one additive (it actually performs several different functions). A few have none at all. Instead, other combinations of different additives perform the multi duty ZDDP functions. If you mix half of each type of oil together then each ones special formula concentration is cut in half and may not work at all.

This is but one example of a very complex formulation that is typical of motor oil. Few actually know the whole formula even within oil companies, they are guarded secrets.

aehaas


Makes sense to me, but I doubt any harm will happen to the engine. JMO

 

[Jason8691]

What if the oils are from the same company? For example, all SOPUS products?

 

[Kruse]

Originally Posted By: Jason8691
What if the oils are from the same company? For example, all SOPUS products?


If you mix Crest and Colgate toothpaste together, will the cavity protection get diluted down? If you mix Tide and Era when you do laundry, will your clothes not come out clean? If you mix two brands of gas, will your injectors clog? I guess it's possible by the same thought as mixing oil brands.

My own uneducated belief is that mixing oils will not harm anything, but then, what do I know?
According to our retired SOPUS expert called Johnny on this forum, Mopar oil is made by two different suppliers depending on if you buy it by the quart or have your oil changed at the dealer from a bulk tank. You could buy Mopar oil and have two different types of oil in your car. I guess they aren't worried.

 

[friendly_jacek]

Originally Posted By: AEHaas
For example, most oils have ZDDP as one additive (it actually performs several different functions). A few have none at all. Instead, other combinations of different additives perform the multi duty ZDDP functions. If you mix half of each type of oil together then each ones special formula concentration is cut in half and may not work at all.


Now, which exactly are the ones without ZDDP, so I will avoid mixing them ;-)