Synthetic vs Conventional Oil: Which One Does Your Car Need?

The short answer

Start with your owner's manual: if it calls for synthetic, use synthetic. If it allows either, both protect the engine as long as the oil meets the spec, but synthetic held up better in AAA's testing and is the stronger pick for heat, stop-and-go traffic, towing and turbocharged engines. Switching between the two is fine, and the oil change schedule still comes from your manual.

You're at the counter or in the oil aisle, and the question is the same: synthetic or conventional? The short answer is that your car already decided part of it for you, and the rest depends on how you drive. Here's how to read what your engine needs and when synthetic is worth choosing even if it's optional.

Does my car need synthetic oil?

Check the owner's manual first. If it says synthetic is required, that's the answer, and conventional isn't a substitute. If it lists a viscosity and a spec without requiring synthetic, you can use either one that meets them.

AAA's own guidance is that few vehicles require synthetic oil, and in most cases drivers can choose between conventional and synthetic oil that meets their vehicle's specifications. That said, some newer engines, especially small turbocharged ones and cars that call for very thin grades like 0W-16, list synthetic or a spec that in practice only synthetic oils meet. Your manual will say. Some European makes also list their own approval codes, which you'll find printed on the back of the bottle when an oil meets them.

Where to look:

• The owner's manual, usually under "Maintenance" or "Specifications," for the grade and any required spec
• The oil filler cap, where many engines show the grade, like 0W-20 or 5W-30
• Your last oil change sticker or invoice, which should list what went in

What do the numbers like 0W-20 mean?

They describe how thick the oil is. The first number with the W (for winter) is how easily the oil flows when it's cold; the lower the number, the faster it reaches the parts at a cold start. The second number is how thick it is once the engine is at operating temperature.

Use the grade your manual lists. A thicker oil isn't "more protection" in an engine designed for a thin one. Modern engines have tight clearances and, in many cases, oil-pressure-controlled parts like variable valve timing that were built around a specific grade. The wrong grade can cause a check engine light on some cars, and it can mean slower oil flow on a cold morning.

Is synthetic oil better than conventional?

In testing, yes. AAA tested 5W-20 synthetic oils from five major brands against conventional oils, and in its 2017 study the synthetics outperformed conventional oils by 47 percent across eight tests. AAA says synthetic could result in less wear over the life of the vehicle.

The same study is careful about the other side: AAA says conventional oil isn't bad for your car, as long as it meets the manufacturer's specifications. So synthetic gives you more margin; conventional is still fine in an engine that allows it.

A quick way to sort it out:

One note on blends: AAA didn't test synthetic blends, because it said the product line is ill-defined. Blends vary by brand, so don't assume one performs like full synthetic.

When is synthetic worth it if my manual says either is fine?

When your driving is hard on oil. AAA says synthetic is well suited to vehicles that operate in extreme heat or cold, sit in stop-and-go traffic, or tow a trailer, and that it benefits newer turbocharged engines.

Around here, that list covers a lot of normal driving. Summer afternoons over 100°F in the valley and I-15 and Railroad Canyon traffic are everyday driving, and plenty of people tow a boat to the lake. If your car fits one or more of those most of the year, synthetic is a reasonable choice even when the manual makes it optional.

Can I switch between synthetic and conventional, or mix them?

Yes, on a car that allows both. You can go from conventional to synthetic and back again at any oil change. If an older engine starts to seep after a switch, have the seals and gaskets looked at; the leak is worth finding either way.

Mixing is fine too. If you're a quart low and the store only has the other type in your grade, add it. Motor oils that meet the same spec are made to work together, and you'll just be running a blend until the next change. Matching the grade on the cap matters more than matching the type.

What to avoid:

1. Using conventional oil in an engine that requires synthetic. It doesn't meet the spec.
2. Going to a different grade than the manual lists because it's what was on the shelf.
3. Stretching the interval on conventional oil to match a synthetic oil's longer range.

Does synthetic oil change how often I need an oil change?

It can, but the manual still sets the schedule. AAA says that regardless of the type of oil, car owners should follow their vehicle manufacturer's recommended oil change schedule. On many cars the oil life monitor does the math for you based on how the car is driven.

Our post on how often to change your oil goes through the normal and severe schedules, oil life monitors and the once-a-year rule. If your car makes mostly short trips, also see whether you need to warm up your car, since short trips are one of the harder jobs oil has.

When to bring it in

Bring it in when the manual's interval or the oil life monitor says it's due, when it's been a year since the last change, or sooner if you're adding oil between changes, see drips under the car or the oil pressure light comes on. A red oil pressure light means pull over when it's safe and shut the engine off.

At our shop, an oil change starts with the right oil for your engine:

1. We fill with the oil type and grade your owner's manual calls for, whether that's conventional, synthetic blend, full synthetic or high-mileage oil.
2. We replace the filter every time.
3. If we spot a leak or anything else while the car is up, you get a written estimate first, and nothing extra is done without your OK.

Repairs come with our 1-year/12,000-mile warranty. To book, call (951) 445-8055.

Recent work like this at our shop

A few recent jobs from our service records. Vehicle, mileage and month only; no customer details.

Vehicle	Work	Mileage	When
2015 Scion tC	Oil and filter change	164,169 mi	Sep 2026
2023 Ford Edge	Oil and filter change	124,143 mi	Sep 2026
2024 Toyota Corolla	Oil and filter change	62,084 mi	Aug 2026
2025 Ford F-150	Oil and filter change	15,973 mi	Aug 2026
2021 Kia Seltos	Oil and filter change	200,411 mi	Jul 2026

Common questions

Is synthetic oil really better than conventional?

In AAA's 2017 testing it was. Synthetic oils outperformed conventional oils by 47 percent across eight tests. AAA also says conventional oil is fine for engines that don't require synthetic, as long as it meets the manufacturer's specifications.

Can I switch from synthetic back to conventional oil?

Yes, if your owner's manual allows conventional oil. The two can be switched at any oil change. If the manual calls for synthetic, stay with synthetic; conventional won't meet the spec your engine was built around.

Can you mix synthetic and conventional oil?

On a car that allows both, topping off with the other type won't hurt the engine. If your engine requires synthetic, top off with synthetic when you can; a quart of the right grade beats running low. Match the viscosity on your oil cap, like 0W-20, and go back to your usual oil at the next change.

Does synthetic oil let me skip oil changes?

It can stretch the interval on cars whose manual allows it, but AAA says to follow the manufacturer's recommended schedule regardless of the type of oil. Our post on how often to change your oil covers the normal and severe schedules.

Should I use high-mileage oil in an older car?

It's an option, not a requirement. High-mileage oils are made for older engines, but the grade and spec in your owner's manual still come first. If your car is leaking or burning oil, the oil type won't fix that; have the leak found.

Sources

• AAA, AAA Engine Oil Study Shows Synthetic Oil Performs Better Than Conventional
• {'AAA Oregon/Idaho, AAA': 'Synthetic Oil Significantly Outperforms Conventional Alternatives | https://info.oregon.aaa.com/aaa-synthetic-oil-significantly-outperforms-conventional-alternatives'}