Mixing synthetic oil and conventional oil

  • Hi Guest, you may choose a LIGHT or DARK theme that works best for you with the "Style Chooser" button at the bottom left on this page!

AdventureWithDanan

Rank VI
Staff member
Moderator
Member

Advocate I

4,459
Safety Harbor, FL, USA
Member #

1358

I’ve heard it said many-a-time that if you mix conventional with synthetic you will have problems. In fact, multiple instructors of mine at Universal Techinal Institute shared this theory. Me personally, I just didn’t think that one oil would act as a caustic agent to the other and break it down...

This article from Amsoil helps dispel that theory with simple data:

CAN YOU MIX SYTHETIC OIL WITH CONVENTIONAL OIL?

The simple answer: Yes. There is no danger mixing synthetic and conventional motor oil; however, conventional oil will detract from the superior performance of synthetic oil and reduce its benefits.

The detailed answer: With synthetic lubricants continuing to grow in popularity, the question of whether synthetics and conventional oils can be safely mixed often arises. I’m guiltyof having mixed different types of motor oil with reckless abandon back in the day. A dash of synthetic blend to top-off my early 90s Buick Century one month and maybe a shot of cheap conventional oil the next.


Here is a link directly to the article:
http://blog.amsoil.com/can-i-mix-synthetic-and-conventional-oil/?_cldee=ZGFuYW5jQGdtYWlsLmNvbQ==&recipientid=contact-1c491e49c6b4e711810e005056827197-a35972c4bb62489b8006fe5ff904a706&utm_source=ClickDimensions&utm_medium=email&utm_campaign=PC Onboarding&esid=a490d390-5b99-4fcf-9e4f-4e1527b5ead2
 

Faded_80

Rank I
Member

Contributor I

203
Seattle
Member #

6969

for what it's worth, most synthetic oils are an API grade 3 synthetic, and not a true synthetic, like most castrol synthetics. grade 3's only have to be 30% synthetic to use the term legally. mix those all you want with shitty oil.

grade 4 synthetics are a poly-alpha-olefin and is a 100% synthetic compound, and has no crude base stock. those cost enough that mixing low grade oil in there is a disservice to the engine and your wallet.

yes, all engine oils will homogenously mix together. if you want to create a middle weight oil between 10w40 and 5w30, mix half and half. it literally doesn't matter to anyone outside of multi-million dollar racing projects.

only thing to really worry about is leaving old oil in the engine too long, as old acidic oil can eat seals, but that's another can of worms.
 

nickburt

Rank VI
Member
Supporter

Advocate III

3,284
Wallasey, UK
Member #

2714

As above, with semi synthetic, do what you like, it's a blend anyway.

Quote from one of the UKs leading oil specialists and suppliers (Opie Oils).

"Semi Synthetic Car Engine Oil
Massive range of semi Synthetic engine oil at great prices
Semi synthetic also commonly known as part-synthetic are 'synthetic blends', which means blends of standard multigrade mineral oil and highly refined ("hydro-cracked") mineral base-stock. These offer great quality at an affordable price, these lubricants are typically what you'll find in your average daily vehicle and extremely popular

Semi / Part synthetic engine oils are designed to have some of the benefits of synthetic oil without matching the cost of pure synthetic fluid."


But, for true synthetic oils:

"Fully Synthetic Car Engine Oil
Massive range of fully synthetic car engine oil at great prices
Opie Oils has a huge range of fully synthetic engine oil available for online ordering and delivery to your home, workplace or garage.

Fully Synthetic engine oil consists of chemical compounds which are not originally present in crude oil but artificially made from other compounds. Fully Synthetic engine oil generally provides superior mechanical and chemical properties than those found in traditional mineral oils and other benefits such as extended oil drain intervals.

Synthetic oils are commonly used in more performance based vehicles, however if you're able to, we always suggest upgrading to one over a semi or mineral (as long as appropriate for you application)."


So given it's a synthetic compound, not found in crude, I agree with @Faded_80 about not doing a disservice to engine or wallet, by "home blending".

Having said that, if you do have to mix for some reason, it's not going to cause a big problem.

What's more important, is the viscosity. Modern engines are built to tighter tolerances than older ones, so a thinner oil is needed. Getting the wrong viscosity will do more damage, more quickly than any other misdemeanor .......

But, again, having said that ... when the needs must ... an unexpected top up out on the trail, just getting something (any viscosity) in there is better than nothing, as long as it's rectified as soon as possible afterwards, and you drive to keep as much strain as possible off the engine.
Same as gearboxes, transfer boxes and diffs. When the needs must ... I've used 10/40 engine oil in a gearbox before now to get out of a situation, but dumped it as soon as possible and refiled with the correct grade.
 
  • Like
Reactions: KJOHNSTER

Mike G

Rank VI
Member

Advocate II

3,741
San Jose, CA
Member #

1334

The shop where I get my oil changed has been offering a 50/50 blend of synthetic and conventional oil for years. With 195k miles on the 4Runner using the blend I've haven't had any issues.

Sent from my SM-T560NU using OB Talk mobile app
 

Longshot270

Rank III
Member

Pathfinder I

I'm gonna put this quote in here again as it is very important.

"What's more important, is the viscosity. Modern engines are built to tighter tolerances than older ones, so a thinner oil is needed. Getting the wrong viscosity will do more damage, more quickly than any other misdemeanor"

Engine oil isn't just for lubrication. Here's some examples, the Jeep renegade 1.4 turbo (gasser) requires high viscosity oil like what is used in diesel applications because valve operation is driven by oil pressure. General motors started playing with the concept of cylinder inactivation and the ones I've seen are in real bad shape. Wikipedia says they started that in the 80s. Dodge 5.7 also started that in the 2004 hemi engine and uses oil pressure to inactivate/activate the valves on 4 out of 8 cylinders during certain low demand conditions. My 2017 Ram has it. Ford has been using oil pressure in their trucks to advance/retard valve timing for over a decade, you can hear plenty of those 4.6 and 5.4 engines with the cam phaser tic owned by people that didn't maintain it. The list keeps going but I'm sure the point is understood.

Changing viscosity will have a direct impact on any of these systems that rely on oil pressure. Engine oil isn't just for engines anymore. For the same reason, many cars require full synthetic because systems like these were designed and rely on the properties and consistency of a full synth. Old style engines can run on just about anything but I don't think it would be wise to do it on a modern car or truck.

That's also why everyone should carry some engine oil, trans fluid and brake fluid 24/7. It won't save you from a catastrophe but may prevent one.
 

Kent R

Executive Director
Staff member
Member
Supporter

Advocate I

4,551
Placerville, CA
Member #

1632

Ham Callsign
K6KNT
Danan thanks for starting this conversation! Rules about oil changes and types of oil used have changed over the years and its hard to keep up, one of the rules that sticks in my brain is once you put synthetic oil in your engine you could never go back to conventional. I now think this is a myth after reading all of the above information. What I know is to maintain your vehicle on a regular basis with high quality lubricants and you will have a better chance of keeping the vehicle going and be able to stay on the trail.
 

jim lee

Rank VI
Member

Advocate I

2,911
Anacortes, WA
Member #

12180

One issue you find people with "older" cars talking about is the Detergent vs Non-detergent oils. By "older" we're talking about early 1960s and older where they typically run straight non-detergent oil. What you hear is that running detergent oils in an old motor like this is basically a death warrant.

So what's the deal?

As the story goes, Non-detergent oils allow sludge to buildup in your engine. Older engines are actually designed somewhat sludge tolerant. For example in my engine the oil pickup actually floats on top of the oil in the oil pan. This is so it will pick up the cleaner oil off the top of the pool. Leaving the sludge behind.

What happens when you run detergent oil in one of these? The detergent starts doing its job and begins to dissolve the sludge. But there is so much sludge that it saturates and starts dropping sludge in places like your oil galleys and channels clogging thing up.. BAM! You oil starve a main or rod bearing and the motor is destroyed.

When I bought my last rebuilt antique truck engine, the builder told me I could run whatever oil I wanted in it. There was no sludge buildup so it didn't matter.

Then of course there's the whole zinc additive thing. But that's another story.

-jim lee
 

Robert Jordan

Rank VI
Benefactor
Member

Advocate I

3,768
Grass Valley, CA
Member #

11582

I will say this as I have seen it multiple times being a mechanic and working on race teams...if you run primarily dino juice and you switch to full synthetic it has the ability to destroy your seals. Now if you have been running synthetic from the start and switch to dino nothing will happen and vice versa if you switch back. It has to do with buildup of deposits and seal deterioration. Synthetics have much more detergents in them and will "clean" your seals up and if by chance you had a small crack or leak at one point and the deposits have sealed them up the synthetic will wash them away leaving you with an issue.
 

Desert Runner

Rank VI
Member

Influencer I

3,857
Southern Nevada
First Name
Jerold
Last Name
Fisler
Member #

14991

I believe the Shell T6 is a grade 4 full synthetic, and i use it in it's diesel form. I went thru so many reviews past the fanboy stage, because it was also a wet clutch compatible oil(ATV), and was known to be extremely resistant to soot buildup. and thus excellent as a diesel oil. Until i started researching, i thought all synthetic oils were full synthetic. I didn't realize the differences that companies could use under that banner. Older engines do not have engine seal compatibility. They will disintegrate. Once rebuilt with new seals, that are of modern materials, older classic engines will be fine with it. Just use a 'zinc' additive if not being modernized with roller tappets. Flat tappets need that zinc in the oil.

Yes i have seen where some manufacturers now spec 0-20 oil. I cringe when i think how thin that is, and what SW desert temps play on engines tolerances. I have also think that is just to thin a oil for the SW desert regions(108-118 degree's atmospheric). 5-30 wt. makes more sense as actual engine bay temps will be 145 degrees or more when operating. I think some manufacturers do allow for alternate grades, depending on use and what environmental temps the vehicle will be operating in.
 
Last edited:

T.Shack

Rank VI
Benefactor
Member

Advocate III

3,676
Red Bluff Ca.
Member #

14817

Following this discussion. Very interesting. I too have always heard never mix the two but then wondered about the semi synthetics. Most of my vehicles I have owned I have always had to go to a thicker oil fb or the summers. Here in the Red Bluff Ca. Area we offer see summer temps up between 110* & 120* .
 

Ditcherman

Rank V
Member

Advocate III

2,515
Sheridan, IN, USA
Member #

3743

If the only disadvantage of mixing mineral based and synthetic is that it drops to the lowest common denominator of protection (crude based), then why are we buying semi synthetic? Do they really do something in in the mix to make it better than just crude based? Sounds like a marketing gimmick to me.
 
  • Like
Reactions: Desert Runner

DanW

Rank VI
Member

Traveler I

2,879
Danville, Indiana
Member #

13615

If the only disadvantage of mixing mineral based and synthetic is that it drops to the lowest common denominator of protection (crude based), then why are we buying semi synthetic? Do they really do something in in the mix to make it better than just crude based? Sounds like a marketing gimmick to me.
Some blends are still outstanding oils. For example, Pennzoil Gold 0w20 in some ways looks even better than Pennzoil Platinum 0w20, on the product data sheet. It has a significantly higher flash point and a virtually identical pour point. Unfortunately, it is hard to find in my area and with rebates on Platinum, I think it has lost it's way in the marketplace.
 
  • Like
Reactions: Ditcherman

Ditcherman

Rank V
Member

Advocate III

2,515
Sheridan, IN, USA
Member #

3743

Some blends are still outstanding oils. For example, Pennzoil Gold 0w20 in some ways looks even better than Pennzoil Platinum 0w20, on the product data sheet. It has a significantly higher flash point and a virtually identical pour point. Unfortunately, it is hard to find in my area and with rebates on Platinum, I think it has lost it's way in the marketplace.
Hey neighbor!
My Tundra is the first thing I’ve really paid attention to and tried to buy the right oil. With about 5 different colors/brands of heavy and farm equipment and diesel and gas trucks, I used to just throw 15-40 rotella in everything. I just don’t want to know and keep track of that knowledge that you have, let alone the inventory. I can say that we recently switched to our local farm Co-op oil and consumption has definitely went down, but I have no idea why. I just wish they made two grades, ‘car’ and ‘heavy’. Antifreeze is almost as bad if not worse.
In other words, how do we know this without being a walking dictionary of oil properties?
 
  • Like
Reactions: DanW and T.Shack

T.Shack

Rank VI
Benefactor
Member

Advocate III

3,676
Red Bluff Ca.
Member #

14817

To me the mixing would only be if you are out & about & oppss you find your oil is low & you forgot or used your back up at or two. Just a a though has anyone had problem with Mobil 1 oils?
 

DanW

Rank VI
Member

Traveler I

2,879
Danville, Indiana
Member #

13615

Hey neighbor!
My Tundra is the first thing I’ve really paid attention to and tried to buy the right oil. With about 5 different colors/brands of heavy and farm equipment and diesel and gas trucks, I used to just throw 15-40 rotella in everything. I just don’t want to know and keep track of that knowledge that you have, let alone the inventory. I can say that we recently switched to our local farm Co-op oil and consumption has definitely went down, but I have no idea why. I just wish they made two grades, ‘car’ and ‘heavy’. Antifreeze is almost as bad if not worse.
In other words, how do we know this without being a walking dictionary of oil properties?
Lol, I'm no expert, just a curious oil nerd. Just stick with an oil made by a reputable name brand, or if a generic, make sure it is SN or SN Plus rated and Dexos 1 Gen 2, and you'll be in great shape. That Co-op oil might be made by Warren Products, which makes a bunch of store brand oils, including Wal Mar Supertech. It is actually very good oil. If not at least API SN, then avoid it.

If you have a direct injected turbocharged engine, go for SN Plus or Dexos 1 Gen 2. Nearly all the big name reputable oils in the appropriate weights meet or exceed those standards, so you'd be good to go.

I run either Mobil 1 or Pennzoil Platinum in my vehicles, at whatever weight the manufacturer specifies. So for my 2018 Jeep, I run Mobil 1 AP 0w20. My 08 JK is currently running Pennzoil Platinum 5w20. I go back and forth between those brands based more on my mood than anything scientific.
 

DanW

Rank VI
Member

Traveler I

2,879
Danville, Indiana
Member #

13615

To me the mixing would only be if you are out & about & oppss you find your oil is low & you forgot or used your back up at or two. Just a a though has anyone had problem with Mobil 1 oils?
I agree.

I've run Mobil 1 in my vehicles for about 34 years or so and never had an issue. That includes cars, trucks, lawnmowers, and more, and every type of automotive engine, from old inline 4 cylinder engines to a 3.5 GDI VVT Turbo Ecoboost. I've never had an issue and engines were running great when I sold them. The longest lived was a 1999 Chevy Silverado 5.3 that I sold to my secretary at about 125k miles. Her husband sold it recently with about 350k miles on it and it still ran great. He continued using Mobil 1 5w30, which I ran in it since about 1k miles on the odometer.

I've run some other oils, including Pennzoil Platinum and Valvoline Synpower, and they were great, too, but I've always been partial to Mobil 1, for just personal preferential reasons. I tend these days to run the higher end Mobil 1 oils, such as AP and EP or High Mileage EP.