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.
Video transcript: How Motor Oil Is Made
Featuring John Gardner, host of Motorhead Garage, and Leonard “Len” Groom, AMSOIL Director of Product Marketing (author profile).
John Gardner: Black gold, bubbling crude, Texas tea. We’ve all heard those terms before. We’re talking about oil. And how do we get it out of the ground from here all the way over to here? There’s a ton of science AMSOIL puts into making that product. Talk a little bit about it.
Len Groom: I’m going to give you the short version. Otherwise, we’re going to need more than a 30-minute show to get this done. Getting from crude requires a refinery. That would be considered your mineral oil, your crude oil: it comes right out of the ground and runs through the refinery. At the refinery, the oil can be cut based on temperature, and it turns into different things, diesel fuel being one of them, gasoline being another. But the base stocks we use, which make up 80 percent of the product, come from that process.
John Gardner: And it gets to this product here. Now, we’re also talking about synthetic and conventional oil. Is it crude or not crude? I know there are a lot of misconceptions there, and I don’t even know. Walk us through that process.
Len Groom: Right. That’s what you have to understand: 80 percent of our product is base oil, not crude oil. To build a synthetic, there are several ways it can be done. But many of the synthetics we use at AMSOIL are the higher-end synthetics. What that means is you take a molecule, if you will. You have to get all the way down to that molecular structure. You take a molecule, you react it with some other things we’re not getting into, and you end up with a brand-new molecule on the other end. There’s your synthetic molecule. You continue to do that over and over and over again. That’s what creates a true, full synthetic base stock.
John Gardner: Now, as a customer, what do I do? Do I put regular oil in, synthetic oil? What are my best choices? There are so many of them.
Len Groom: There are so many different products out there. There are blends; there are conventionals. AMSOIL obviously specializes in high-end full synthetic products, so you’re going to find me recommending synthetic every single time. But as you know, synthetics are becoming table stakes. They’re showing up in the owner’s manual. Because of the performance levels of some of these engines and their requirements, synthetics are becoming necessary.
John Gardner: I’m all about buying the best products I can possibly buy. Just go to amsoil.com, and you’ll find them all there.
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?
- Are the API group classifications progressively better?
- Are Group III base oils “synthetic?”
- Are synthetic base oils magic?
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: 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: 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: 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: 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: 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.
Base oil groups at a glance
| API group | How it’s made | Saturates | Sulfur | Viscosity index |
|---|---|---|---|---|
| Group I | Solvent-refined mineral oil | Less than 90% | More than 0.03% | 80–120 |
| Group II | Hydroprocessed mineral oil | 90% or more | 0.03% or less | 80–120 |
| Group III | Severely hydrocracked (reconstructed molecules) | 90% or more | 0.03% or less | 120 or higher |
| Group IV | Polyalphaolefins (PAO), chemically engineered | Synthesized base stocks; not defined by these limits | ||
| Group V | Everything else: esters, polyglycols, silicones and others | Varies by chemistry | ||
Source: API 1509 base oil interchangeability guidelines.
Paraffinic vs. naphthenic base stocks
Mineral base oils are classed as paraffinic or naphthenic, based on how their hydrocarbon molecules are arranged.
Paraffinic stocks are mostly long, straight hydrocarbon chains. They resist oxidation, have high flash points and a high viscosity index, so their viscosity stays steadier as temperature changes. Their drawback is wax: untreated, it turns solid in the cold, so refiners remove the heaviest waxes and usually add pour-point depressants.
Naphthenic stocks are rich in ring-shaped hydrocarbons. They contain little or no wax, so they stay liquid at low temperatures, but they thin considerably when hot and generally have a low viscosity index. They’re also stronger solvents. Oil is generally labeled naphthenic when its paraffinic carbon content is below about 55–60%.
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.
Some 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 1999 advertising dispute between two major oil companies changed that. One company challenged the other’s marketing of a Group III-based motor oil as “synthetic.” The National Advertising Division of the Council of Better Business Bureaus ruled that the Group III oil, as then formulated, could be called a synthetic.
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.
This article is part of our Motor Oil Basics guide. See every article in the guide.
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Start with AMSOIL synthetic motor oil and find the exact AMSOIL products for all your vehicles with the AMSOIL Product Guide.
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Brad Nelson is an AMSOIL Technical Writer II who develops content about lubricant technology, product performance and vehicle maintenance. He works with engineers and product specialists to translate testing, application requirements and approved performance claims into clear customer guidance. Nelson’s work includes AMSOIL Blog articles, AMSOIL Magazine features and technical explanations concerning motor oils, gear lubricants, vehicle systems and maintenance practices.
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