Key Takeaways
- Synthetic oil is chemically engineered for more uniform molecules, offering superior protection in extreme heat and cold.
- Conventional oil is refined from crude petroleum and remains appropriate for many older or simpler engine designs.
- Synthetic oil typically allows longer oil change intervals, often 7,500–15,000 miles depending on the vehicle.
- Your owner's manual is the definitive source for which oil type and viscosity grade your engine requires.
- Switching from conventional to synthetic is generally safe, but consult your manual before changing oil types.
- Cost per change is higher for synthetic, but the longer intervals can narrow the real-world price gap.
Option A
Synthetic Motor Oil
The engineered, high-performance formulation.
Best for: Best for modern engines, extreme temperatures, high-mileage vehicles, and drivers who want longer intervals between oil changes.
Option B
Conventional Motor Oil
The time-tested, refined petroleum option.
Best for: Best for older engines with straightforward designs, moderate climates, and vehicles whose manufacturers specify it.
If you drive a modern vehicle or a turbocharged engine
Synthetic Motor Oil
Modern engines, especially turbocharged ones, run hotter and tighter tolerances demand the superior thermal stability and lubrication synthetic oil provides.
If you own an older vehicle whose manufacturer specifies conventional oil
Conventional Motor Oil
Many engines built before the mid-2000s were designed and calibrated around conventional oil; switching without checking the manual adds unnecessary risk.
If you drive in extreme climates — very cold winters or very hot summers
Synthetic Motor Oil
Synthetic oil flows more readily at low temperatures and resists breakdown at high temperatures, protecting the engine during cold starts and sustained heat.
If minimising per-visit maintenance cost is a priority
Conventional Motor Oil
Conventional oil costs less per quart and per change. For drivers with lower annual mileage on appropriate engines, it can be the more economical choice.
If you want to extend time between oil changes
Synthetic Motor Oil
Synthetic oil is engineered to resist oxidation and thermal breakdown longer, allowing many manufacturers to approve intervals of 10,000 miles or more.
What Makes These Two Oils Different
At the most fundamental level, the difference comes down to origin and refinement. Conventional motor oil is refined directly from crude petroleum. The refining process removes many impurities, but the resulting molecules are still somewhat irregular in size and shape. Synthetic motor oil, by contrast, is chemically synthesized — its molecules are engineered to be uniform, which translates to more consistent performance across a wider range of conditions.
Both types contain base oil plus an additive package — detergents, anti-wear agents, friction modifiers, and viscosity improvers. The distinction is in the base oil quality and how it behaves under stress. Synthetics maintain viscosity (their resistance to flow) more reliably when temperatures swing from a frigid winter morning to a hot summer highway run. Conventional oils can thin more at high heat and thicken more in deep cold, which is where protection can temporarily lapse. See our guide to fluids under your hood for how oil fits into the broader picture of engine care.
| Criterion | Synthetic Motor Oil | Conventional Motor Oil |
|---|---|---|
| Base source | Chemically engineered molecules | Refined crude petroleum |
| Cold-weather performance | Flows readily at low temps | Can thicken in extreme cold |
| High-heat stability | Resists breakdown at high temps | More prone to thinning under heat |
| Typical change interval | 7,500–15,000 miles | 3,000–5,000 miles |
| Cost per oil change | Higher upfront cost | Lower per-change cost |
| Turbo engine suitability | Recommended or required | Generally not recommended |
| Older engine compatibility | Compatible with most modern seals | Traditional choice for older engines |
Performance, Protection, and Real-World Conditions
For most drivers in moderate climates with standard engines, conventional oil provides adequate protection when changed on schedule. The problems arise at the edges: extreme cold, sustained high heat, long highway hauls, towing, or stop-and-go city driving that never lets the engine fully warm up. Under those conditions, synthetic oil's engineered stability delivers measurable advantages — faster lubrication at startup (when most engine wear occurs), better resistance to oxidation, and a slower rate of viscosity breakdown over time.
Turbochargers are a particularly important consideration. Turbo bearings spin at extremely high speeds and are bathed in hot oil even after the engine shuts off. Conventional oil can coke (bake into a residue) in those passages; most turbocharged engine manufacturers now specify full synthetic for this reason.
~90%
New US vehicles specifying synthetic or synthetic-blend oil
Industry estimates suggest the vast majority of new passenger vehicles sold in the US now come factory-filled with full synthetic or synthetic-blend motor oil.
Up to 40%
Engine wear occurring at cold startup
Automotive engineers commonly note that a significant share of engine wear happens in the first moments after a cold start, before oil fully circulates — an area where synthetic's faster flow provides an advantage.
2x–3x
Longer drain interval vs. conventional
Many full-synthetic formulations are approved for intervals roughly two to three times longer than conventional oil under normal driving conditions, according to vehicle manufacturer specifications.
Oil change intervals are directly tied to oil type. Conventional oil typically calls for changes every 3,000–5,000 miles under normal conditions, though that 3,000-mile figure is widely considered outdated for modern vehicles. Synthetic oil is generally rated for 7,500–15,000 miles depending on the vehicle and manufacturer. Our article on oil change intervals breaks down exactly how those numbers are set. Part of what appears to be a higher upfront cost for synthetic often evens out when you factor in fewer changes per year — though the math varies by vehicle and driving pattern, as detailed in our car ownership costs overview.
How to Choose — and What to Watch Out For
The single most reliable guide is your vehicle's owner's manual. It specifies the oil viscosity grade (such as 5W-30 or 0W-20) and whether synthetic is required, recommended, or optional. Following that specification protects your engine and, in many cases, your warranty. Automakers have increasingly moved to full synthetic as the factory-fill standard, particularly for vehicles produced in the past decade.
If your manual permits either type, consider your driving conditions: extreme temperatures, high mileage, or a turbocharged engine all tip the scales toward synthetic. If your vehicle is older and has been running conventional oil without issues, there is generally no harm in continuing — or in making a clean switch to synthetic at your next change, provided your manual doesn't advise against it.
One note worth dispelling: the old idea that switching to synthetic "causes leaks" in older engines is largely a myth rooted in older synthetic formulations. Modern synthetics are compatible with the seals used in virtually all engines. For more on outdated maintenance rules, see our piece on car maintenance myths that cost drivers money. And if you're deciding whether to handle an oil change yourself or leave it to a shop, our DIY vs. shop guide can help you weigh the options.
Synthetic Blends: A Middle Ground
Synthetic-blend motor oil mixes conventional base oil with a portion of synthetic base stock. It offers better performance than straight conventional oil — particularly at temperature extremes — at a price point between the two. Blends are a reasonable choice for drivers whose manuals permit either type but who want some of synthetic's benefits without the full cost premium. As always, check your owner's manual for the grade and type your engine is designed for.