Most synthetic motor oil sold in the US is made one of three ways: crude oil is torn apart and rebuilt with hydrogen under heat and pressure (Group III), ethylene gas is chained together into polyalphaolefins (Group IV PAO), or natural gas is turned into wax and then into oil (gas-to-liquid). The base oil that comes out is then blended with an additive package.
The word "synthetic" describes the process, not where the carbon came from. That surprises people. A Group III synthetic and a conventional oil can start from the same barrel of crude.
What is actually in a bottle of oil
A quart of motor oil is base oil plus additives. The base oil does the lubricating. The additives fight acid, stop foam, hold soot in suspension, lower the pour point and keep the viscosity stable as the oil heats up (Amsoil).
Neither Amsoil nor Mobil publishes an additive percentage. What they do publish is the base oil story, and that is where the manufacturing differences live.
The American Petroleum Institute sorts base oils into five groups. The group is decided by three lab numbers: how much of the oil is saturated hydrocarbons, how much sulfur is left, and the viscosity index, which is how little the oil thins out with heat (Machinery Lubrication).
API group | Saturates | Sulfur | Viscosity index | How it is made |
|---|---|---|---|---|
Group I | Under 90% | Over 0.03% | 80 to 120 | Solvent refining of crude |
Group II | Over 90% | Under 0.03% | 80 to 120 | Hydrotreating or hydrocracking of crude |
Group III | Over 90% | Under 0.03% | Over 120 | Severe hydrocracking at higher heat and pressure |
Group IV | Not defined | Not defined | Over 120 | PAO synthesized from ethylene in a chemical plant |
Group V | Not defined | Not defined | Not defined | Everything else: esters, PAGs, silicones |
Figures from Machinery Lubrication's summary of the API 1509 base oil definitions. Groups III, IV and V are what US makers sell as "full synthetic."
Conventional oil: distill and clean
Conventional oil is separated out of crude, not built. The refinery heats crude in a distillation column, pulls off the fraction that boils in the lubricant range, then uses solvents to strip out wax and aromatics (Machinery Lubrication).
The trouble is what gets left behind. Crude contains thousands of different molecule shapes and sizes. Amsoil's blog puts it plainly: base oils distilled from crude contain several different molecules that are detrimental to lubrication. Some burn off early. Some turn to sludge. Some are just sulfur.
That mix is why a Group I oil thins more with heat, evaporates more and oxidizes sooner. The additives have to work harder to cover for it.
Group III: tear it apart, rebuild it
Group III is how most "full synthetic" on a Walmart shelf is made. The feedstock is still crude. The difference is what the refinery does to it.
Hydrocracking runs the crude fraction over a catalyst with hydrogen gas at high temperature and pressure. The hydrogen breaks the ring-shaped and unsaturated molecules and saturates them into straight and branched chains. A second step, usually called hydroisomerization or wax isomerization, rearranges the long waxy chains so they stay liquid in the cold. Machinery Lubrication describes Group III as the same hydrogen process as Group II, just run more severely at higher heat and pressure.
The result tests like a synthetic. Over 90 percent saturates, almost no sulfur, viscosity index above 120. Molecules are far more uniform than anything distillation can produce.
Whether this counts as "synthetic" was settled in 1999. Castrol had quietly switched its Syntec oil from PAO, which it had been buying from Mobil, to a hydrocracked Shell base oil in late 1997 or early 1998. Mobil complained to the National Advertising Division of the Better Business Bureau. In April 1999 the NAD let Castrol keep the word synthetic on the label (Noria forum summary). Amsoil's blog notes Mobil's lab testing of Syntec samples from December 1997 found they contained 100 percent mineral oil. Since that ruling, any Group III oil sold in the US can be labeled synthetic.
Group IV: build it from gas
PAO is the one most people picture when they hear "synthetic." Nothing is distilled. Ethylene, a two-carbon gas, is reacted into a longer alpha olefin such as decene, then those are linked into chains of a controlled length. Amsoil describes it as chemically synthesized, built from uniform molecules, typically ethylene.
Because every molecule is designed rather than sorted, PAO has no wax to begin with, no sulfur and a naturally high viscosity index. Pour points are very low, which is why PAO-heavy oils dominate 0W grades and arctic service.
The drawback is cost. Hydrocracked base oil is significantly cheaper to make than PAO (Noria), which is why most shelf synthetics are Group III. Mobil 1 is the best known PAO brand, but Mobil's FAQ page does not disclose how much PAO is in current Mobil 1, only that its base stocks are better than those in the Mobil Super blend. Treat any PAO percentage you read on a forum as a guess.
Gas-to-liquid: natural gas to wax to oil
Shell's PurePlus process, used in Pennzoil Platinum and Ultra Platinum, starts with natural gas instead of crude. Shell makes it at the Pearl GTL plant in Ras Laffan, Qatar, a joint venture with Qatar Petroleum, and says it is the only company making base oil from natural gas at commercial scale (Hydrocarbon Processing, 2014).
The chemistry is Fischer-Tropsch. Methane is reacted with oxygen to make syngas, a mix of carbon monoxide and hydrogen. The syngas is scrubbed so it does not poison the catalyst, then built up over a catalyst into hydrocarbons ranging from gases to a thick wax. Isodewaxing, hydrocracking and distillation turn that wax into base oil (Amsoil blog on GTL).
Shell says the base oil is crystal clear because it carries fewer of the impurities found in crude. Amsoil, a competitor, concedes any GTL oil counts as fully synthetic because the base oil is synthesized, but argues the end product is similar to other synthetics and formulation matters more than origin. Both are selling something, so read both with that in mind.
In API terms, GTL base oil is usually filed as Group III or "Group III+." It is made from gas but classified by its lab numbers, not its feedstock.
Does the manufacturing method show up in the engine
Yes, as a group. AAA ran eight ASTM bench tests on API-licensed GF-5 oils and found the synthetics beat conventional oils by 47 percent on average across shear stability, deposits, volatility, cold pumpability and oxidation (AAA). AAA did not break results out by base oil group, so that figure does not tell you PAO beats Group III.
The price gap is smaller than most people think. AAA put a do-it-yourself conventional change at about $28 for five quarts against $45 for synthetic, and a shop change at $38 against $70. Walmart currently lists a 5-quart jug of Mobil 1 Advanced Full Synthetic 5W-30 at $31.97.
What you cannot do is tell the base oil group from the label. "Full synthetic" on a US bottle can mean Group III, GTL, PAO or a blend of all three. The API donut and the ILSAC starburst tell you it passed the tests, which is what the engine cares about.
Check your manual
Your owner's manual lists the viscosity grade and the spec the oil must meet, such as API SP, ILSAC GF-6 or GM dexos1. The grade is often printed on the oil fill cap too. Match those. Whether the base oil came from crude or gas is not something the manual asks for.
What to do today
- Open the owner's manual and write down the grade and spec, for example "0W-20, API SP, ILSAC GF-6A."
- Look for the API donut and ILSAC starburst on the back of the bottle. If the spec is there, the manufacturing route does not matter for a warranty.
- Ignore "100% synthetic" or "PAO-based" claims unless the maker publishes a data sheet backing them. Mobil, for one, does not publish a PAO percentage.
- If you drive in sub-zero winters, pick a 0W grade. Those are where PAO and GTL base oils make a measurable difference.
- Price it per quart. The AAA gap of about $17 per DIY change is the real cost of synthetic.
Sources
- Machinery Lubrication: Base Oil Groups Explained
- Amsoil blog: Synthetic vs Conventional Oil
- Amsoil blog: Is Motor Oil Made From Natural Gas Better?
- Amsoil blog: Are All Synthetic Oil Groups the Same? Group III vs IV vs V
- Noria forums: Hydrocracked Basestocks Affecting an Industry
- Hydrocarbon Processing: Shell Starts Commercial US Output of Motor Oil Made From Natural Gas
- Vehicle Service Pros: Pennzoil's PurePlus Technology
- Mobil 1 Product FAQs
- AAA Oregon: The Truth About Oil Changes
- Walmart: Mobil 1 Advanced Full Synthetic 5W-30, 5 Quart
Featured image: SHELL OIL REFINERY MERSEYSIDE by Terry Kearney (CC0). Source
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