Short answer: check your manual first, because on many modern cars the decision has already been made for you. A growing number of manufacturers specify synthetic outright, and on those vehicles this is not a choice you have. Where you do have a choice, synthetic buys a longer interval between changes and better behavior at temperature extremes.
Whether it saves you money depends on cost per mile and how many shop visits you avoid, not on what the two bottles cost side by side, and that is the comparison almost nobody selling oil will run for you.
First: do you actually have a choice?
Open the manual before you open your wallet.
Manufacturers specify an oil grade, and increasingly they specify a type. Modern engines with tight tolerances, turbochargers, or long factory service intervals are frequently designed around synthetic, and using conventional in an engine that specifies synthetic is not a saving, it is a deviation from specification that can affect both engine life and warranty coverage.
If your manual specifies synthetic, the rest of this article is background reading rather than a decision. If it specifies conventional, or allows either, read on.
What genuinely differs
The technical account is consistent across every source on the topic, including the oil manufacturers themselves, and it is not controversial.
Conventional oil is refined from crude. Its molecules vary in size and shape, and that variation is where its limits come from.
Synthetic oil is engineered, with more uniform molecules and a stronger additive package. The practical consequences: it resists thermal breakdown better, so it holds its lubricating properties longer under heat; it resists oxidation better, which is the slow degradation that happens regardless of load; and it flows better when cold, which matters most in the first minute after a cold start, when most engine wear occurs.
Synthetic blends are what the name suggests, a mixture, sitting between the two on both performance and price.
That is the whole of the chemistry you need. Where it gets less honest is what people do with it.
What it changes for your interval, and why the numbers disagree so much
Here the sources stop agreeing with each other, and it is worth seeing the spread rather than an average of it.
For conventional, published guidance runs from roughly 3,000 to 5,000 miles at the shorter end up to around 5,000 to 7,500 depending on who is writing. For synthetic, Advance Auto Parts and others put the common manufacturer recommendation around 5,000 to 7,500 miles, note that many models are designed for 10,000, and observe that some go as far as 15,000. Mobil, Midas, Take 5 and Midway all publish figures in broadly this territory with meaningful variation between them.
Two things to take from that. First, synthetic genuinely does extend the interval, and every source agrees on the direction. Second, the size of the extension is not a fact about oil, it is a fact about your engine, which is why your manufacturer’s number is the one that counts. Notice also that nearly every page publishing these ranges sells oil or oil changes.
Our guide on how often you really need an oil change covers how to find your own interval rather than adopting a range.
The value question nobody answers
If synthetic costs more per change but goes further between changes, the price per bottle tells you nothing useful. Here is the arithmetic to run with your own numbers.
- Take the total cost of one conventional change, parts and labor, and divide it by your conventional interval in miles. That is your cost per mile on conventional.
- Do the same for synthetic, using your manufacturer’s synthetic interval, not the longest number you have read.
- Compare the two figures.
- Then add the thing the arithmetic misses: how many fewer trips to a shop, and what your time is worth.
We publish no prices here, deliberately, because prices vary by region and date and any figure printed on a page like this is wrong somewhere. Run it with the numbers you were actually quoted. The answer is frequently closer than the bottle price suggests, and on cars with a long synthetic interval it often comes out in synthetic’s favor.
Where synthetic earns its price regardless
Independently of the arithmetic, there are conditions where the performance difference is doing real work:
- Cold climates. Better cold flow means oil reaches the top of the engine faster after a cold start, and that first minute is when wear happens.
- Sustained heat. Hot climates, towing, mountain driving, or anything that keeps the engine working hard for long periods.
- Short-trip driving. Counterintuitively, the pattern that is hardest on oil, because the engine never fully warms and contaminants accumulate rather than boil off.
- Turbocharged engines. Turbos run extremely hot and are demanding on oil, which is a large part of why so many turbocharged cars specify synthetic in the first place.
If two or three of these describe your driving, synthetic is doing more than extending an interval.
Switching an older engine
The old worry was that switching a high-mileage engine to synthetic would cause leaks. The usual explanation given is that synthetic’s better cleaning action clears deposits that were sitting in worn seals.
The honest version is more measured. On a healthy engine, switching is not a problem, and there is nothing about synthetic that damages seals. On an engine that already has deteriorated seals and is being partly held together by its own deposits, a switch can reveal a leak that was already there. Revealing is not causing, though it feels the same from the driver’s seat.
High-mileage formulations exist specifically for this, with additives intended to condition seals. If you are switching a car with substantial mileage and unknown history, that is a reasonable middle path to discuss with whoever changes your oil.
What this does not change
Switching to synthetic does not let you ignore your car’s own guidance. If your vehicle has an oil life monitor, it is running its algorithm on your driving regardless of what is in the sump, and our guide on what the oil life monitor is actually measuring covers how much weight to give it.
It also does not change the time-based half of the interval, and it does not substitute for the rest of the schedule. Oil is one line on a longer list, laid out in our car maintenance schedule by mileage.
Frequently asked questions
Do I have to use synthetic oil?
If your manual specifies it, yes. A growing number of modern engines, particularly turbocharged ones and those with long factory service intervals, are designed around synthetic and specify it outright.
How much longer does synthetic actually last?
Published guidance commonly puts synthetic intervals around 5,000 to 7,500 miles, with many models designed for 10,000 and some going considerably further. The extension depends on your engine, not on the oil alone.
Is synthetic worth the extra money?
Compare cost per mile rather than cost per bottle, using your own quoted prices and your manufacturer’s intervals for each. On cars with a long synthetic interval it often comes out favorably, and fewer shop visits count too.
Will switching to synthetic cause leaks in an old engine?
Synthetic does not damage seals. On an engine with already deteriorated seals, a switch can reveal an existing leak. High-mileage formulations exist for this situation.
What is a synthetic blend?
A mixture of synthetic and conventional oil, sitting between the two on both performance and price. It is a genuine middle option rather than a marketing label.