What Is Synthetic Oil Made From?

PAO, esters, and gas-to-liquid base oils explained plainly, why “synthetic” doesn’t legally mean what most drivers assume, and which base stocks AMSOIL actually builds its lineup on. US & Canada, 2026.

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20+ years running AMSOIL across cars, trucks, motorcycles, and diesel equipment — this is the guide I wish existed when I first started asking why two bottles both labeled “full synthetic” could be built so differently.

Last updated 2026. Reviewed against the API base oil classification system and published base stock chemistry from PAO, ester, and GTL manufacturers.

On This Page

1. Quick answer: what’s actually in the bottle
2. The API base oil groups (I through V)
3. PAO: the original “true synthetic”
4. Esters: the polar performer
5. Gas-to-liquid (GTL): synthetic from natural gas
6. Why Group III oil can legally be called “synthetic”
7. The other 20-30%: what’s in the additive package
8. What base stocks AMSOIL actually uses
9. Alan’s take
10. Frequently asked questions

Quick Answer

Synthetic motor oil is built from one of three engineered base oil chemistries — PAO (polyalphaolefin), synthetic esters, or gas-to-liquid (GTL) base oil — each produced through a controlled chemical process rather than simply refined from crude, blended with a package of detergents, dispersants, anti-wear agents, and viscosity modifiers that typically makes up 20-30% of the finished bottle.

The catch: a 1999 advertising-industry ruling opened the door for oils made from heavily processed Group III mineral oil to legally carry the word “synthetic” on the label too, even though Group III starts from refined crude rather than true chemical synthesis. That’s why reading the base oil chemistry, not just the word “synthetic,” is the only way to know what’s actually in the bottle.

The Classification System

The API Base Oil Groups (I Through V)

Every motor oil on the shelf starts as one of five API-classified base oil types. Groups I and II are conventional mineral oils, simply refined from crude with varying degrees of purity. Groups III, IV, and V are what get marketed as “synthetic,” though as you’ll see below, they earn that label in very different ways:

GTL (gas-to-liquid) base oils are typically classified as Group III or an unofficial “Group III+” — more on where they actually fit further down this page.

Group IV

PAO: The Original “True Synthetic”

Polyalphaolefin (PAO) is built, not refined. The process starts with ethylene gas, which is polymerized into 1-decene, then chemically joined into long, highly uniform hydrocarbon chains through a controlled process called oligomerization. The result is then hydrogenated to strip out remaining double bonds, locking in stability.

Because PAO molecules are built from scratch rather than sorted out of crude oil, they come out remarkably uniform, with no wax and no sulfur or phosphorus contaminants. That uniformity is exactly why PAO-based oils perform the way they do:

• Excellent cold flow: no wax means no crystallization at low temperatures, so PAO stays fluid well below -40°F.
• High thermal stability: the uniform molecular structure resists breaking down under sustained heat.
• Strong shear stability: PAO holds its viscosity rating under the mechanical stress of engine operation better than refined base stocks.

The one known weakness: pure PAO is chemically inert enough that it can actually shrink rubber seals slightly, which is why virtually every PAO-based oil on the market blends in a small percentage of esters, whose polarity swells seals back to a proper fit.

Group V

Esters: The Polar Performer

Synthetic esters are made through esterification, a chemical reaction between an organic acid and an alcohol. Unlike PAO, ester molecules contain oxygen atoms, which gives them a natural electrical polarity. That single property is responsible for nearly everything esters are prized for:

• Metal-seeking behavior: the polar molecules are attracted to metal surfaces and cling there, forming a protective film that can persist even for a moment after oil pressure drops.
• Seal conditioning: esters naturally swell rubber seals, offsetting PAO’s tendency to shrink them, which is why the two are so often paired.
• Highest thermal stability of any base stock class, along with some of the lowest pour points, making esters the go-to for extreme-temperature and racing formulations.

Pure 100% ester oils exist but are rare in mainstream automotive products; most formulations blend esters into a PAO base rather than using esters as the sole base stock.

The Third Path

Gas-to-Liquid (GTL): Synthetic From Natural Gas

A newer route to a synthetic-grade base oil starts with natural gas instead of crude. The Fischer-Tropsch process, developed in 1920s Germany, converts natural gas into “syngas” (a mix of carbon monoxide and hydrogen), which a cobalt catalyst then reassembles into long-chain hydrocarbon waxes. Shell’s Pearl GTL facility in Qatar is the best-known large-scale example of this process feeding directly into motor oil production.

Because GTL base oils are built from simple, extremely pure gas molecules rather than sorted out of crude, they land close to PAO in real-world performance, even though they’re typically classified as Group III (or the unofficial “Group III+”) rather than Group IV. This is genuinely one of the more reasonable arguments for calling Group III-derived synthetics legitimate — GTL just doesn’t fit neatly into the PAO-vs-Group-III framing most drivers have heard.

Where the Confusion Comes From

Why Group III Oil Can Legally Be Called “Synthetic”

In the late 1990s, Castrol began selling an oil made from Group III base stock under the name Syntec Full Synthetic. Mobil filed a complaint with the National Advertising Division (NAD) of the Council of Better Business Bureaus, an industry self-regulatory body, not a court of law, arguing that Group III oil was simply highly refined crude and that calling it “synthetic” was misleading.

In April 1999, the NAD ruled in Castrol’s favor, finding a reasonable basis for the claim based on industry practice and expert testimony. The ruling had no legal force beyond the NAD’s own self-regulatory authority, but it set the industry norm that’s still followed today: “synthetic” is treated as a marketing term describing performance properties, not a guarantee of production method.

Beyond the Base Oil

The Other 20-30%: What’s in the Additive Package

Base oil, whether PAO, ester, GTL, or Group III, typically makes up 70-80% of the finished bottle. The rest is an additive package doing distinct, specific jobs:

These additive categories are largely the same across synthetic and conventional oils. What changes is how much work they have to do. Because a synthetic base oil is already purer and more thermally stable on its own, the additive package can focus on enhancing performance rather than constantly fighting the weaknesses of the base stock underneath it.

Where AMSOIL Fits

What Base Stocks AMSOIL Actually Uses

AMSOIL built its reputation as an early PAO adopter, and its Signature Series lineup is formulated around Group IV PAO blended with Group V esters, rather than the Group III base stock that’s become the industry default since the 1999 NAD ruling. In practical terms:

• The PAO base gives the cold-flow and thermal stability benefits described above.
• The blended-in esters offset PAO’s seal-shrinking tendency and add the metal-seeking film strength esters are known for.
• This combination is why AMSOIL rates Signature Series for extended drain intervals rather than sticking to the same schedule as a Group III-based “full synthetic.”

That doesn’t make every Group III synthetic a bad choice; a well-additized Group III oil at a standard interval is a perfectly reasonable option for a lot of drivers. But if the extended-drain claim on a bottle is part of why you’re buying it, it’s worth knowing which base stock is actually earning that claim.

Want the PAO/ester base stock instead of Group III? Get AMSOIL Signature Series at wholesale pricing as a Preferred Customer.

Alan’s Take

Frequently Asked Questions

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