Oil compatibility myths

[VQLT]

After everything I have read on BITOG I'm a believer that you can mix different brands, types, viscosity, etc. with no problem. My question is, where did all the myths originate regarding "you can use different viscosity but must be same brand" or "you cant mix synthetic and conventional" or "different additive formulas will interact and cause sludge" have any of these so called myths every been proven true?

 

[cci]

I'd like to learn more about this too.

While there may be some legitimate concerns in some cases, I just mixed 20w-50 and straight 60 from the same manufacturer and it worked great.

 

[SOHCman]

Some manufacturers say right on the bottle, Amsoil comes to mind, that you must lower the use interval and not go the advertised milage on the oil if you mix. Sure they say they are compatible, but want you to keep it in the family for maximum protection...

I'm not dissing Amsoil, and I think it's reasonable for a company to NOT guarantee the performance of a competitors oil... makes sense. Just poured some Amsoil in my mower actually

 

[VQLT]

If I mixed synthetic and conventional I would base my oil change interval on the conventional oil. Like a lot of others on this forum I buy oil when on sale or clearance therefore I have several brands and viscosity ( I always stay within the viscosity range recommended by the manufacture)
I'm very curious when people say different additives are not compatible. how many different additives are there? why would then not be compatible? Just one of the many myths (IMHO) that i'm trying to discover if there is any merit

 

[MolaKule]

Most all modern PCMO's and HDEO's will mix.

Now Mixability and Compatibility are two different concepts.

In the earlier days of refining, crudes came from all kinds of sources, some good, some not so good, and refining techniques and refining controls weren't as sophisticated as they are today.

Add to this those disparate additive packages and you did not know what might happen if those oils were mixed.

Today, base oils are more highly refined and the additive packages more technologically advanced.

As SOCHman points out, certain PCMO and HDEO oils should not be mixed and in his example mixing a full synthetic with a mineral oil of unknown additive package and base oil may reduce performance.

 

[d00df00d]

I can see getting sludge if your mix contains an oil that can't stand up to your application. But that's because that oil is wrong, not because it had a poor interaction.

Other than that, my understanding is that you can mix all day and nothing will blow up right away; you just can't expect the mix to perform optimally. Maybe it will, maybe it won't. It depends on how their additive packages interact. The problem is that you can't really know that unless you test it correctly, which is basically impossible for the average consumer.

Oils from the same product line are be more likely to have the same additive chemistry, which makes them more likely to work well together when mixed.

I can see people misinterpreting all of that as "you can only mix the same brand" or "additive clash will cause sludge" or whatever. But that's a misinterpretation.

 

[compratio10_5]

I suppose that some of these "myths" come from SAE papers over the years that show low temperature pumpability problems with various viscosity modifiers and used oil properties. The compatibility of most new oils are tested but there have been some problems with mixing of various additives with used oil. If your vehicle starts at ambient temperatures below zero F, review the abstract of some of the following SAE papers: 2012-01-1708, "In Service Low Temperature Pumpability: Field Performance vs Bench Tests", 2008-01-2481,"Thermal History of the Engine Oil and Its Effects on Low-Temperature Pumpability and Gelation Formation", 2004-01-1932,"In Service Low Temperature Pumpability of Crankcase Lubricants-Effect of Viscosity Modifiers", 2000-01-2942, Deterioration in Used OIL Low Temperature Pumpability Problems". Good Reading.

 

[VQLT]

if both oils are high quality oils with proper API rating it would seem that the end result would still be a high quality oil. You had mentioned in your post about additive packages interact. I know you don't want to mix ammonia and bleach, but oil additives? aren't after market oil additives (STP, etc.) compatible with all oils? why don't their additives interact? I don't want to go off on a tangent so basically I just want to find out how the myths got started and is there any proof / validity

 

[benjy]

there are sooo many variations of oil today + surely the pro's know best, so i believe buying what you want is far better than "wingin it"!!!

 

[krismoriah72]

most of the myths come from 'expert' mechanics..

The oldest is that you need to use the same brand or bad things will happen.. the engine gets 'used' to a brand.

I have mixed about every brand and weight in the last 5 years of clearance oils from my stashes.

If you are asking if you can mix some loose bottles and make a frankenbrew...do it..

The only oil failure you are going to see is lack of..

 

[VQLT]

The oldest is that you need to use the same brand or bad things will happen.. the engine gets 'used' to a brand.

How did a myth like this get started? was is a marketing campaign to make sure you did not buy a competitors oil or is there a factual basis for the myth

 

[MolaKule]

Originally Posted By: VQLT
The oldest is that you need to use the same brand or bad things will happen.. the engine gets 'used' to a brand.

How did a myth like this get started? was is a marketing campaign to make sure you did not buy a competitors oil or is there a factual basis for the myth


I never saw any marketing to that effect.

Most myths and misinformation eminate from a misunderstanding of oil chemistry and breathing too much exhaust fume.

 

[Shannow]

a) it's not a myth.

By the new BITOG standard, "nothing will blow up"...well not likely, but we've seen a couple of instances over the years on BITOG where they have caused issues.

As to the nothing blown up, there are a couple of instances of oil not playing nicely together. One was the residual factory fill on a bunch of Toyotas not playing with the first service fill. The PPDs and VMs didn't play well together and the oil gelled, ultimately at room temperature, and the engines died due to oil starvation...

Rare ?

Of course.

Impossible ?

Definitely.

Issues are most likely to occur if you genuinely NEED that 0W or 5W to adequately lubricate your engine and start playing oil chemist...live here in Oz, and you can get away with anything.

Here's what ASTM D6922 ACTUALLY offers with regard to "mixability" of oils..
https://bobistheoilguy.com/forums/ubbthreads.php/topics/3503439/The_oft_quoted_ASTM_D6922


Mix them, cool them, heat them, cool them again, and they aren't supposed to split like salad dressing, or drop chunks of solids...that's it...and with a handful of reference oils, not every fully formulated oil on the shelf.

No performance tests...nothing.

Before the advocates of mixing wade in with their "pile of failed engines" strawman, which they create over and oer again.

Nobody can show to date ANY scientific publication that suggests in the first place that it's good practice to mix...that says a lot.

Many manufacturers state that their oil will MIX per ASTMD6922, but recommend against it...that's not just marketing, these guys make (and blend) oil.

In their eagerness to justify and recommend their own "blend" (it's a mix, not a blend)...they have
* pointed to the ASTM D6922 that oils are "compatible"...they ARE, just within the limits of the standard...it does NOT confer that all the engine tests, CCS, MRV etc. are going to work.
* pointed to the Appendix E base oil interchangability rules and stated that this advocates mixing within those groups and ratios and that the standard tests are met if these guidelines followed.

Both laughable "justifications" for the practice.

 

[4wd]

Originally Posted By: VQLT
The oldest is that you need to use the same brand or bad things will happen.. the engine gets 'used' to a brand.

How did a myth like this get started? was is a marketing campaign to make sure you did not buy a competitors oil or is there a factual basis for the myth


This is the kind of stuff you hear around an oil change business … and change that PP based on time regardless of miles or what Mopar engineers put in writing …

 

[dwendt44]

The mixability/compatibility 'myth' goes way back to the 50s and before.
I recall that back in the 60s, many oils started listing
military specs for mixability on their containers.
For a long time, there really wasn't a problem.
Now though, with 0W-16, conventional, blends and synthetics,
C spec oils, etc... who knows.
Likely nothing bad would happen short term.
With many 'bad' oils are questionable outlets, I'd be
darn careful about thoughtless mixing.


My 2¢

 

[4wd]

Originally Posted By: VQLT
The oldest is that you need to use the same brand or bad things will happen.. the engine gets 'used' to a brand.

How did a myth like this get started? was is a marketing campaign to make sure you did not buy a competitors oil or is there a factual basis for the myth


I tend to follow the anything in moderation concept … when I have a few “maverick” quarts on the shelf … I toss one or two in with the same brand …maybe another grade. For example I grabbed the last two jugs of Pennzoil Platinum 5w30 on clearance and they had a few Pennzoil Platinum 0w40 quarts even cheaper … by using the quarts I can do two full OCI’s with that ratio … what’s wrong with that …
(5+1 in 6 quart motor)

 

[toneydoc]

I tend to believe a little mixing goes on with each oil change.
ie there is always some amount of dirty, used oil that remains in an engine at oil change
Then we add new, fresh oil.
The amount that remains in the engine is probably highly variable
Soooo, there is always a small amount of mixing. However it appears that the fresh oil
and new additives successfully mix with the residual oil and the engine carries on
This is not the same manner you fellas are talking about but this does happen every oil
change.
Mixing oil, yes I do it some but not all the time. Mainly to use up stock of oil

 

[demarpaint]

I've mixed oil in beaters to get rid of widow and orphan quarts of oil. I guess my success falls into the engine didn't blow up camp. It is not a practice I advocate, and I don't mix oils in anything I don't caterogize as a beater. At the moment my fleet contains no beaters. Having said that I have seen some very good used oil analysis posted over the years with different brands and/or grades mixed. If one were to believe in used oil analysis, and that used oil analysis tell the condition of oil and how good or bad it is doing, then mixing isn't a problem according to those reports. Me I will continue not to mix in anything but a beater. It just isn't worth it to me.

 

[Donald]

At one point in time when seals where not made of the same materials they are today, switching from conventional oil to synthetic could cause a problem. One that often went away on it's own.

 

[Shannow]

Originally Posted By: Donald
At one point in time when seals where not made of the same materials they are today, switching from conventional oil to synthetic could cause a problem. One that often went away on it's own.


Seal compatibility was largely understanding how synthetic lubricants dealt with seal materials.

A bit like additive solubility, and the early synthetics needing a bit of "carrier oil".