Are all Synthetic Oil Groups the Same? Group III vs IV vs V

The simple answer

No. In fact, there are wide performance differences between base oil group categories. Generally speaking, Group IV base oils offer the best performance, Group III second best, and so on in reverse order. But be forewarned – there are exceptions. And, you can’t judge motor oil performance solely on base oil type.

You must take into account its entire formulation, including the additives.

base oil groups

The detailed answer

To ease your study of the topic, we’ve broken it down into the following common questions:

What are the different base oil groups?

The American Petroleum Institute (API) developed a classification system for base oils that focuses on the paraffin and sulfur content and degree of saturation of the oil. The saturate level indicates the level of molecules completely saturated with hydrogen bonds, leaving them inherently un-reactive.

Translation: they’re more resilient to chemical degradation, meaning they last longer and perform better.

There are five groups in the classification system, ranging from Group I – Group V:

Group I Characteristics
Group I base oils are the least refined of all the groups. They are usually a mix of different hydrocarbon chains with little uniformity. While some automotive oils use these oils, they are generally used in less-demanding applications.

• Group II Characteristics
Group II base oils are common in mineral-based motor oils. They have fair-to-good performance in the areas of volatility, oxidation stability, wear prevention and flash/fire point. They have only fair performance in areas such as pour point and cold-crank viscosity.

• Group III Characteristics
Group III base oils consist of reconstructed molecules that offer improved performance in a wide range of areas, as well as good molecular uniformity and stability. Manufacturers can use these synthesized materials in the production of synthetic and semi-synthetic lubricants.

• Group IV Characteristics
Group IV base oils are made from polyalphaolefins (PAO), which are chemically engineered synthesized base oils. PAOs offer excellent stability, molecular uniformity and improved performance.

• Group V Characteristics
Group V base oils are also chemically engineered oils that do not fall into any of the categories previously mentioned. Typical examples of Group V oils are esters, polyglycols and silicone. As with Group IV oils, Group V oils tend to offer performance advantages over Groups I – III. An example of a mineral-based Group V exception is a white oil, a very pure lubricant used in industries ranging from cosmetics to food processing.

Are the API group classifications progressively better?

In other words, is a motor oil made from Group III base oils better than one made from Group II base oils, and so on?

In general, yes. Unlike your food, which generally gets less healthy the more it’s processed, base oils offer improved performance as the level of refinement/processing increases.

But there are side cases that smash that rule of thumb.

base oil groupsSome motor oils made from Group III oils can outperform some Group IV motor oils. That’s because the final formulation is a function of the base oils and additives working in tandem. Like base oils, additives come in a range of qualities. So you could have a Group III oil with top-shelf anti-wear, anti-oxidant and other additives that outperforms a Group IV motor oil, even though Group IV base oils provide more pronounced benefits than Group III base oils. The point is, a motor oil can’t be judged solely by its base oils – you need to take the entire formulation into account.

Then we have the Group V category, which is a sort of catch-all for anything that doesn’t fit into the other four groups. In fact, some Group V oils are completely unsuitable for automotive use.

Are Group III base oils “synthetic?”

Yes, in most countries anyway.

A true definition for the term “synthetic oil” has been difficult to reach, although it has generally been accepted that the term represents those lubricants that have been specifically manufactured for a high level of performance. Group III base oils with very high viscosity indices can be called synthetic oils in most countries.

Historically, it was widely accepted that only Group IV base oils made from PAOs were true “synthetics.”

A famous lawsuit between Mobil and Castrol changed that. Mobil charged that Castrol was falsely marketing its Syntec motor oil as a synthetic oil although it wasn’t made from PAO base oils. Mobil’s claim was based on results of independent lab testing that showed samples of Syntec it obtained as early as December 1997 contained 100 percent mineral oil.

The two sides battled it out, but in a landmark 1999 ruling, the National Advertising Division of the Council of Better Business Bureaus ruled that Castrol Syntec, as then formulated, was a “synthetic” motor oil.

Debate raged then, and still rages today. You can find all kinds of purists populating Internet forums who refuse to recognize Group III oils as “synthetics.” For them, it’s PAO or nothing.

Try not to get caught up in the “my-base-oil-versus-your-base-oil” cage match. The base oils that go into the oil aren’t as important to your engine as the performance that comes out of the oil. Look for motor oils that offer performance claims backed by industry-standard testing or real-world results. That’s what’s really important.

If you really need to know which base oils a formulation uses, you’ll have to do some investigative work since oil companies protect that information as proprietary.

For details, check out this post: How Much “Synthetic” Is In My Oil?

Are synthetic base oils magic?

Ok, that’s not what people really ask. But many falsely think synthetic base oils are not refined from crude oil and that switching to only synthetic lubricants could drastically reduce our dependence on foreign oil and non-renewable sources. If synthetic base oils aren’t made from crude oil, from what raw material are they made? Unicorn horns and rainbow dust?

Synthetic base oils are made from crude. But they’re much more highly refined than conventional base oils. The chemical reaction process used to make synthetic base oils removes the impurities inherent to conventional base oils, such as sulfur and waxes. This results in a higher-performing product that’s much better for your engine.

Science, not magic.


  1. You need to search youtube, there are crap oil filters out there, Fram is one of them. Oil is the lifeblood of an engine, full synthetic is cheap, and its additive packages exceed that of conventional oil, not to mention the same viscosity of conventional vs group III or group IV full synthetic has a better flow rate at low and high temps. I went full synthetic in the late 80s and will never go back. Supertech full synthetic can be had from $13.99 to $17.99, I ran 15,000 mile oil changes with that stuff in turbos with a filter at 7500 miles no sludge build up in the engine or any engine related problems. Full synthetics are simply better. They were back in the 1980s when shop mechanics were against full synthetic. Look up flow tests on Youtube, conventional oil vs synthetic oil, if you don’t give a crap about your car, fine, put conventional in it, but for $15 for a 5 qt jug of Supertech 10w30. Here’s a lab test no one can argue with to get you started.

  2. I’ve been doing my own mechanic work all my life, I’m 68 now. Small engines, motorcycles,outboards, cars, trucks,aircraft, industrial machines, tractors, etc.. I’ve owned very few “new” vehicles and I usually push every possible mile I can out of every one I have owned. That’s usually about 250k miles in 10 – 20 years before getting rid of them (cars and trucks). I’ve yet to ruin anything because of “bad oil” or even been able to tell any difference whatsoever in “performance” because of differences in brands, types, conventional or synthetic. Generally speaking as long as you don’t run out of oil you’re going to be fine with whatever you have in there as long as it hasn’t become horribly contaminated or not pushed more than about 15k miles before changing. The number one enemy of your machine is how you operate it. That accounts for about 99.9% of how reliable and long lasting it is going to be. Preventative maintenance, obsessing over oil and fuel brands and technicalities, etc. is about the remaining .1%. For oil I think the best advice is just be sure to go with whatever meets the mfg. spec., change it about every 5k miles with a new filter and you’ll be fine. Can someone explain to me how you can tell the “performance” difference in one oil or another from a general consumer’s perspective ? This same comment goes for spark plugs, oil filters, additives, etc. on the shelves of auto parts stores.

  3. I use Amsoil products but it just seems like,and I know this part has been beaten to death, that the company is making excuses concerning their engine oil makeup. What prompted me to make this statement is that on the blog it’s like I can see the writing on the wall about your base oil even before I’ve finished reading. It’s like we are being lead up to an excuse about Group lll performance with a top notch additive package outperforming Group lV. People want a majority Pao product when they are paying top dollar. This is a demand that I see all over the internet forums and it doesn’t seem to want to go away.Please listen to what the people want. If I go to a bakery and order a cake that’s supposed to contain whipped cream then I want Real whipped cream! I’m paying for it so that’s what I should get. I would be very disappointed to find out that I got Ready Whip instead. Does that analogy make sense? Please reply. Thanks.

    1. Hi capri,

      I understand your point, and I understand your frustration. There is a select group of enthusiasts out there who seem to want only PAO-based synthetic motor oil. My question is, why? Why do they want PAO-based oil? Obviously, because they want an oil that offers the best protection and performance. As said in the blog post, base oil alone doesn’t determine oil performance. Sure, it greatly influences the oil’s performance, but one must consider the entire formulation, which includes additives.

      We aim to make the best synthetic oils on the market, and we publish test results to back that claim. We don’t disclose the base oils we use in particular products because it’d give our competitors an unfair advantage. Does Coca-Cola reveal its secret recipe? I suspect just about every other oil company you contact will tell you the same thing.

      I invite you (and anyone else) to look at the test results we’ve published showing the performance of AMSOIL synthetic motor oils. You can find them here: Compare the results to those of other oils (if you can find any). Continue to use the products in your vehicles. If they don’t meet your expectations, you’re free to try another brand.

      However, I think you’ll be hard-pressed to find another oil that performs better. Thanks for your loyalty, and I hope AMSOIL products continue to meet your expectations.


    2. Hi, Group III is not outperforming all of the group IV, at least not in the lab. AMSOIL and Redline are group IV, I believe the rest in this lab test is group III, final result has the group IV’s having the best additive packages. Pennzoil Ultra Platinum comes in 3rd making it the best bang for the buck. However, all oils tested here are way better than conventional oil.

  4. When you get an answer like “Amsoil “gave to “Ottomatic” you can bet they use the inferior Class III base stock and are not really synthetic oil at all! I have researched synthetic oil for over 35 years, and what has happened is they got the law changed so that Class III oil, which is highly refined crude oil can be classified as “synthetic”. This way they can sell you crude oil for a highly inflated price of synthetic! I used both over the last 35 years and the Class III is inferior!

  5. I think that the base oil and the additive package are two different things.
    The Gp IV base oils performance (oxidation resistance, VI, cleanness) definitely better like the Gp III

  6. I see that Pennzoil has just started advertising the 1st ever synthetic motor oil made from natural gas. Being as PAOs are made from natural gas, this is obviously a lie.

  7. So is Amsoil motor oil Group III or IV? That question seems to be overlooked in this blog.
    I also searched Amsoil’s website for the answer….no luck there either.
    And since you mentioned additives, what does Amsoil put in their oil? A specific list would be most helpful.

    1. Hi Ottomatic,

      As noted in the post, AMSOIL (and every other oil company) maintains its formulations as proprietary. Disclosing specific formulation details would give our competitors an advantage. Also, as noted in the post, how the oil performs is more important that which base oils and additives it contains. Ultimately, that’s what your engine cares about.



    2. It shouldn’t be a secret whether the base oil a company starts out with is grade III or grade IV. Keep your secrets about additives but at least say what base oil you start with.

  8. Really good info about the grouping of oils that you’ve put into this article, I learned a ton! I especially liked how you said that it’s important to understand what your engine needs in terms of oil and not fighting over “my oil is purer than your oil”. My brother is looking to buy a diesel truck and I will let him know the differences!

  9. Nice over view. But myself, and probably most others, would like to see a (fairly comprehensive) list of motor oils and their group number. All the major US brands like Quaker, Napa, castrol, Valvoline, Pennzoil, Rotella, Brad Penn, RoyalP, Mobil, Amsoil, Redline, etc etc

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