Synthetic vs Conventional Oil: What Your Car Actually Needs

The right oil for your car is whatever your owner's manual specifies: a viscosity grade (like 0W-20) and a performance standard (like API SP or a carmaker spec such as dexos1). Whether that oil is synthetic or conventional matters less than whether it meets that spec. That said, synthetic oil has real advantages, and many modern engines effectively require it. Here is how to tell which camp your car is in.

• If your manual says synthetic is required, use synthetic. This is common on turbocharged engines, most European cars, and many engines that call for 0W-16, 0W-20 or 0W-8 oil.
• If your manual lists a viscosity and an API/ILSAC spec and says nothing about synthetic, a conventional or blend oil that meets the spec is acceptable. Synthetic is an optional upgrade.
• If you tow, drive in extreme heat or cold, do lots of short trips, or want longer intervals, synthetic is usually worth the extra cost even where it is optional.

Whatever you buy, the two things on the label that must match your manual are the viscosity grade and the specification. The base oil type comes third.

All motor oil is a base oil plus an additive package (detergents, dispersants, anti-wear compounds, friction modifiers, antioxidants, viscosity modifiers).

Conventional oil uses base oil refined from crude oil. Its molecules vary in size and shape, and it contains more impurities. It works well in normal conditions but thickens more in the cold, thins and oxidizes faster in heat, and breaks down sooner.

Synthetic oil uses base oils that are more heavily refined or chemically built (hydrocracked Group III, PAO Group IV, or ester Group V stocks). The molecules are more uniform. In practice that means:

• Better flow at cold start, when most engine wear happens.
• Better resistance to oxidation and thermal breakdown at high temperatures, including inside turbochargers.
• Fewer deposits and less sludge over time.
• Lower volatility, so less oil burns off between changes.

Synthetic blend (semi-synthetic) is a mix of conventional and synthetic base stocks. There is no legal minimum for how much synthetic it contains, so quality varies by brand. It is a middle ground in price and performance.

Open the "Specifications" or "Maintenance" section of your owner's manual and look for three things.

1. Viscosity grade

Written like 5W-30. The first number with the W is cold (winter) performance; lower flows better cold. The second number is thickness at operating temperature. Use exactly the grade listed. Some manuals give a temperature chart allowing more than one grade; pick the one that matches your climate.

2. Performance standard

Look for one or more of these:

• API service category such as API SP (the current gasoline standard as of this writing; older manuals may list SN or SM, which current oils exceed). The API "donut" symbol on the bottle shows it.
• ILSAC such as GF-6A or GF-6B (the "starburst" symbol). GF-6B is specifically for 0W-16 and thinner oils.
• Carmaker specs such as GM dexos1, Volkswagen VW 504 00 or 508 00, Mercedes-Benz MB 229.5 or 229.71, BMW Longlife-01 or LL-17 FE+, Porsche A40, Ford WSS-M2C.... If your manual names one of these, buy oil that lists it on the label. Using a non-approved oil on a car under warranty can create problems if an engine claim is ever disputed.

3. Whether synthetic is required

Manuals for engines that need synthetic usually say so directly. Many oils that meet European specs or 0W grades are synthetic by necessity, even if the manual does not use the word.

Synthetic is a good choice, even on a car that does not require it, if any of these apply:

• Turbocharged or supercharged engine. Turbo bearings run very hot, and conventional oil can coke (bake into deposits) there.
• Gasoline direct injection (GDI). Many GDI engines are prone to low-speed pre-ignition (LSPI) and fuel dilution; current API SP and dexos1 Gen 3 oils address LSPI, and most of those oils are synthetic.
• Severe service. Towing, hauling, mountain driving, extended idling, rideshare or delivery work, dusty roads, or mostly short trips under about 10 miles where the engine never fully warms up.
• Extreme climates. Regular sub-zero starts or sustained 100°F+ heat.
• You want longer intervals. Synthetic tolerates the long end of your manual's interval or oil-life monitor reading better than conventional does.
• Older engine with sludge history. Some engines are known for sludging; synthetic's detergency and stability help, though it won't undo existing damage.

Conventional oil remains a reasonable, cheaper choice for an older, naturally aspirated engine that calls for 5W-30 or 10W-30, is driven moderately, and is changed on schedule.

"Synthetic causes leaks in older engines"

Synthetic oil does not damage seals. Modern synthetics are formulated with seal conditioners. What can happen is that a high-mileage engine with already-hardened seals and deposits that were plugging small gaps starts seeping after a switch. The leak was already there. If you are worried, a "high mileage" oil (conventional or synthetic) includes extra seal conditioners.

"Once you go synthetic, you can't go back"

You can switch back and forth freely. Conventional and synthetic oils are compatible and mix without harm. Mixing just gives you the properties of a blend.

"Synthetic lets you skip oil changes"

Synthetic lasts longer, but the additives still deplete and the oil still collects fuel, moisture and soot. Follow your manual or the oil-life monitor, not the marketing on the bottle. See how often you should really change your oil.

"Thicker oil protects better"

Not in modern engines. Tight tolerances, variable valve timing and hydraulic lifters are designed around a specific viscosity. Running a heavier grade than specified can slow oil reaching the top of the engine at startup and can trigger variable timing codes such as P0011 on some engines.

A synthetic oil change typically costs more than a conventional one, often by a noticeable but not dramatic amount (typical estimate, varies by vehicle, oil capacity and region). Because synthetic usually allows intervals at the longer end of your manual's range, the cost per mile can end up close to conventional.

A quick way to compare:

1. 1Note the price of each type of change at your shop or for a DIY oil and filter.
2. 2Note the interval you would actually use for each (from your manual or oil-life monitor).
3. 3Divide price by miles.

Plug your own numbers into the maintenance cost calculator or the oil change interval estimator to see the yearly difference for your driving.

A checklist for the parts store aisle:

• Viscosity matches the manual exactly (for example 0W-20, not 5W-20, unless the manual allows both).
• API donut shows the current service category (SP or newer), or ILSAC starburst is present.
• Any carmaker approval in your manual is printed on the label (dexos1, VW 504 00, MB 229.71, etc.).
• Enough quarts for your engine's capacity with filter (check the manual; many four-cylinders take 4 to 5 quarts, many V6 and V8 engines take 5 to 8).
• A filter that fits your engine and is rated for the interval you plan to run.

Write down the date, mileage, oil brand, grade and spec at every change. If you ever need to make a warranty claim on an engine, receipts showing the correct oil at the correct intervals are your best evidence. Under the federal Magnuson-Moss Warranty Act, a manufacturer generally cannot void your warranty just because you used an aftermarket or non-dealer oil change, as long as the oil meets spec; the FTC explains your rights here (opens in a new tab). Logging each change in your free CarManager garage keeps those records with the car and shows them on your owner history report when you sell.