Synthetic vs Conventional Oil: Most "Synthetic" Is Crude

The usual answer on synthetic vs conventional oil is that synthetic is built molecule by molecule in a laboratory and conventional comes out of the ground. It's a tidy story and it's mostly wrong.

Most oil sold as full synthetic in the United States is refined crude. No federal rule governs the word, and an advertising industry panel accepted calling it synthetic in 1999, and the reasoning behind that decision tells you more about what you're buying than any brochure does.

What a bottle actually promises is printed on the back label, never the front.

What do the two words actually mean?

Base oil, the part that's not additive, is sorted by API into five groups. Where a finished oil sits in that table decides most of what it can do.

Groups I and II are refined crude, and Group II is what "conventional" means on an American shelf today. Group III is the same crude taken much further: hydrocracked hard enough to reach a viscosity index of 120 or more.

Group IV is polyalphaolefin, genuinely assembled from small molecules. Group V is everything else, mostly esters, usually blended in rather than sold alone.

Those are the five base stock groups API 1509 defines. Group III and Group IV both carry the word synthetic on American shelves, and Group II and Group III start from the same crude.

The line people imagine runs between Group II and Group IV. The line that the label draws runs somewhere in the middle of Group III, and it was put there by an advertising dispute.

Why is Group III allowed to say synthetic?

Because an advertising panel decided in 1999 that it could, and the decision still stands. In March 1999 Mobil complained to the Better Business Bureau's National Advertising Division that Castrol Syntec, made from hydrocracked petroleum rather than constructed molecules, shouldn't be sold as synthetic.

The NAD found for Castrol. Leaning on industry references, it held that Castrol had a reasonable basis for the claim, because hydrocracking makes the base oil through an intended chemical reaction.

So synthetic became a marketing term rather than a technical one. A bottle marked full synthetic may hold Group III, Group IV, or a mixture.

The bottle won't tell you which, and no rule requires it to.

Synthetic vs conventional oil: what is the measurable difference?

Marketing claims about protection are hard to check. One property isn't, and it's the one that separates the two most cleanly.

NOACK volatility asks how much oil evaporates in an hour at 250°C, which is roughly what the oil above the pistons experiences. The test is ASTM D5800 and the result is a percentage of weight lost.

Read the last two rows together. API lets an oil lose 15%, and GM's dexos1 allows about 13%, or 12.5% in its current edition by published summaries.

Typical conventional oil sits above 13%.

So a manual that asks for dexos1 has ruled out most conventional oil without mentioning synthetic at all.

Evaporation isn't an abstraction, either. Oil that boils off leaves the oil pan, which is one of the routes by which an engine appears to burn oil, and what stays behind is heavier than the grade on the bottle, a useful reminder that the grade is a promise about the oil when it's new, not a fixed property of it.

0W-20 vs 5W-30 follows what that promise is actually worth.

What does synthetic actually buy you?

Three things, and they're worth different amounts depending on how you drive.

Worth noticing first how differently manuals word it. Of the manuals we read, the 2020 Nissan Altima says a synthetic 0W-20 of the right grade may be used, which is permission rather than a requirement.

The 2020 Equinox goes further and asks for full synthetic oils that meet dexos1. The 2023 Hyundai Sonata asks for full synthetic at API SN PLUS or above, and puts a lesser oil on the severe-service schedule.

So where a maker asks for synthetic, it's asking for a set of test results too, and synthetic is the route to passing them, with three real advantages over conventional oil:

• Cold starts. Synthetic base stock stays fluid at lower temperatures, so oil reaches the top of the engine faster on a winter morning. Most engine wear happens in those seconds.
• Heat tolerance. Lower volatility and better oxidation resistance mean it degrades more slowly under sustained load: towing, mountains, hot climates, small turbocharged engines.
• Interval. The extended drain interval on a modern car assumes an oil that survives it. That assumption is part of what the price buys.

If your driving is short trips in a mild climate in a naturally aspirated engine, the margin is real but you'll never see it. If it is a turbocharged three-cylinder in traffic, you're using every bit of it.

What doesn't it buy you?

Three claims that follow this subject around and don't hold up.

It doesn't clean a neglected engine. Synthetic oil carries detergents; so does conventional oil.

Neither dissolves hardened sludge, and an oil that suddenly loosened a large deposit would be doing you no favors.

It doesn't cause leaks in older engines. This one has a real origin: some seals shrink in contact with PAO, a common synthetic base stock, and modern synthetics are formulated with seal compatibility in mind.

What actually happens is that an old, hard, already-leaking seal keeps leaking, and the oil that was in the engine when you noticed gets the blame.

There's no commitment. You can switch to synthetic and back again, in either direction, at any oil change.

No flush is needed and nothing is locked in.

What is the difference between full synthetic and a blend?

A synthetic blend is exactly what the name says: synthetic and conventional base oils mixed deliberately, priced between the two. It's a ratio, sold openly.

The question people actually mean when they ask about the difference between synthetic and full synthetic is usually this one: is the blend enough? The answer is the same as everywhere else on this page.

If the blend carries the specification your manual names, it's enough. If it doesn't, the word on the front doesn't rescue it.

And since the blend is a mixture by design, it settles a related worry: mixing synthetic and conventional oil in your own engine isn't dangerous either. The industry bottles the mixture.

Is it worth the money?

Usually yes on a modern engine, though what decides it is the specification on the bottle rather than the word synthetic. Synthetic costs roughly two to three times conventional per quart, which on a five-quart change is a difference measured in tens of dollars, once or twice a year.

Set against that: a modern turbocharged engine runs its oil hotter than a 1990s four-cylinder did, service intervals have stretched, and the oil is doing more jobs than lubrication alone. On some engines it operates variable valve timing, and on a few it is the environment a timing belt lives in.

None of this is printed on the front of the bottle. What is printed, and which mark actually decides whether an oil suits your engine, is in how to read an oil bottle.

The framing that holds up is specification versus no specification, rather than synthetic versus regular oil. Find the code your manual names, buy an oil that carries it, and the base oil question answers itself, because the specification already answered it.

How do you pick the oil for your own car?

We don't tell you to buy synthetic, and we don't tell you it's a waste. The manual settles it, and we couldn't find a case where a general recommendation beats the code printed in the book for your own engine.

It comes down to one code in the manual, and the first step is finding out whether yours names one. To choose between synthetic and conventional oil for your own car, work through the 4 steps below, in order:

1. Open the manual and find the specification. A code such as dexos1, WSS-M2C970-A1 or VW 504.00, not just a grade. How to read an oil bottle shows where it appears on the back label.
2. Buy an oil that carries that code. If only synthetics carry it, that's your answer and it didn't require an opinion.
3. If the manual names only a grade, buy conventional. It is a perfectly sound arrangement: change it on the conventional schedule and nothing is being given up.
4. Don't buy synthetic and then stretch the interval to pay for it. That trades the margin you just bought for the money you just spent.

Common questions

Asked most often

What is the difference between synthetic oil and regular oil?

Processing, mostly: both usually begin as crude, and synthetic base stock is refined much harder, or in the case of Group IV built from small molecules. The measurable result is lower evaporation loss, better cold flow and slower degradation under heat.

What is the difference between synthetic and full synthetic?

Nothing consistent, because neither term is regulated: in practice "full synthetic" is used to distinguish a product from a synthetic blend. The specification on the back tells you more than either phrase.

Is synthetic oil worth it?

If your manual requires a specification that conventional oil can't meet, the question doesn't arise. If it doesn't, it depends on your climate, your engine and your intervals, and the answer can legitimately be no.

Can I switch back to conventional oil?

Yes, at any change, provided the oil still meets what the manual asks for. Nothing about synthetic is one-way.

Does synthetic oil last twice as long?

It degrades more slowly, but the interval is set by the manufacturer for a whole system, not by the oil alone. Follow the manual's figure rather than the bottle's implication.

General information about oil specifications, compiled August 2026 for the US market. Specification limits are revised periodically; your owner's manual and the manufacturer's current approved-oils list are the authority for your engine. See the disclaimer.