Synthetic vs Conventional Oil: What Actually Differs
Not laboratory versus crude — almost all of it is crude. One measured number separates them, and it's the number your manual is quietly using.
Short answer
Both usually start as crude oil. The difference is how hard it was processed.
The useful gap between synthetic and regular oil isn't chemistry versus nature. It's measured performance, and the clearest measure is how much oil boils away at engine temperature. Conventional loses more than 13%. Synthetic loses 9 to 11%. That single number is why some manufacturer specifications rule conventional oil out without ever using the word.
Ask this question on the internet and you get the same answer everywhere: synthetic is built molecule by molecule in a laboratory, 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. It has been legal to call it synthetic since 1999, and the reasoning behind that decision tells you more about what you're buying than any brochure does.
What 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.
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 Group III is allowed to say synthetic
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. Consulting chemical reference works, it concluded that a substance need only be substantially changed to count as synthetic, and hydrocracking clears that bar comfortably.
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.
Worth knowing
This is less cynical than it sounds. A good Group III oil outperforms a mediocre Group IV one, because the base stock is only half the product. The additive package is the other half. Buying by group would be no smarter than buying by label. Buying by specification is the thing that works.
The difference you can actually measure
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.
| Oil | Typical NOACK loss | What it means |
|---|---|---|
| Conventional passenger car oil | Over 13% | More of it leaves as vapor; what remains is thicker than what you poured in |
| Synthetic passenger car oil | 9–11% | Less loss, so the oil in the oil pan at 5,000 miles is closer to the oil you bought |
| API SP / ILSAC GF-6A limit | 15% maximum | The floor every licensed oil must clear |
| GM dexos limit | 13% maximum | Tighter than API, and this is where it gets interesting |
Read the last two rows together. API lets an oil lose 15%. GM's dexos specification allows 13%. Typical conventional oil sits above 13%.
So a manual that asks for dexos1 has ruled out most conventional oil without mentioning synthetic at all. That's the mechanism behind every "your car requires synthetic" conversation at a counter, and almost nobody explains it that way.
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.
What synthetic actually buys you
Three things, and they're worth different amounts depending on how you drive.
- 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 it doesn't 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 early synthetics used ester base stocks that shrank certain seals, and that was decades ago. 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.
Full synthetic, synthetic blend, and the difference between them
A synthetic blend is exactly what the name says: synthetic and conventional base oils mixed deliberately, priced between the two. It's not a compromise product in any dishonest sense. 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 sump isn't dangerous either. The industry bottles the mixture.
Is it worth the money?
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.
The honest framing isn't synthetic versus regular oil. It's specification versus no specification. 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.
What to do about it
- Open the manual and find the specification. A code such as dexos1, WSS-M2C948-B or VW 504.00, not just a grade. How to read an oil bottle shows where it appears on the back label.
- Buy an oil that carries that code. If only synthetics carry it, that's your answer and it didn't require an opinion.
- If the manual names only a grade, conventional oil changed on the conventional schedule is a perfectly sound arrangement.
- 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
What is the difference between synthetic oil and regular oil? Processing, mostly. Both usually begin as crude; 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.
Sources
- API 1509 Annex E: base oil interchangeability guidelines — the definitions of Groups I to V used here, including the saturates, sulfur and viscosity-index boundaries that separate Group II from Group III. American Petroleum Institute
- “Synthetic” as the word relates to motor oil — account of the March 1999 National Advertising Division decision on Mobil's complaint against Castrol Syntec, which is why Group III can be sold as synthetic in the US. Porsche Club of America
- API SP and ILSAC GF-6 category requirements, including the 15% maximum NOACK volatility limit. American Petroleum Institute
- The NOACK volatility test, ASTM D5800 — method and the typical ranges quoted here for conventional and synthetic passenger car oils. BG Products laboratory
- GM's 13% maximum NOACK requirement for dexos, tighter than the API limit. Published dexos specification summaries
- What we didn’t do. We didn’t test any oil, and we didn’t buy the ASTM or API documents — both sit behind paywalls, so the limits above are taken from published summaries rather than read off the standards themselves. The NOACK ranges for typical conventional and synthetic oils come from a laboratory that also sells oil treatments, which is a real interest to declare; we quote the ranges because the test method is public and repeatable, not because the source is disinterested. And we have deliberately given no brand recommendation, because we have run no comparison that would justify one.
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.