Conventional and synthetic oil both lubricate your engine, but they are made differently and behave differently. The best choice is usually the one your manufacturer specifies.
The short answer
Use the oil type and viscosity grade listed in your owner’s manual. If your manual allows more than one option, synthetic oil generally offers stronger protection and longer intervals but costs more per change. Conventional oil costs less up front and is still suitable for some engines and light driving.
What is conventional oil, and what is a conventional oil change?
Conventional oil is refined from crude oil and combined with additives. A conventional oil change is simply the standard service using that type of oil: the old oil is drained, the filter is replaced, and the engine is refilled with conventional oil. The steps are the same as any other oil change, as described in Oil Changes 101. Only the oil differs.
What are synthetic and synthetic blend oils?
Synthetic oil is engineered with more uniform molecules and a tailored additive package. It is designed to resist breakdown at high heat and to flow well in the cold.
Synthetic blend oil mixes synthetic and conventional base oils. It sits between the two in performance and typically in price, and it is sometimes marketed as an affordable upgrade.
Differences at a glance
| Conventional | Synthetic blend | Full synthetic | |
|---|---|---|---|
| Source | Refined crude oil | Mix of both | Engineered base oils |
| Cost per change | Lowest | Middle | Highest |
| Typical interval | About 3,000 to 5,000 miles | About 5,000 to 7,500 miles | About 7,500 to 10,000 miles or more |
| Hot and cold performance | Adequate | Better | Generally best |
The intervals are commonly quoted ranges, not rules. See how often to change your oil for details.
Cost: upfront vs. over time
Synthetic changes usually cost more at the counter, and prices vary by shop, region, and the amount of oil your engine holds. Because synthetic can allow longer intervals, the cost per mile may be closer than it first looks. It depends on whether you actually stretch the interval, so compare quotes rather than relying on a general figure.
Which should you use?
- Follow the manual. Many newer engines, especially turbocharged ones, require synthetic or a specific oil specification. Using the wrong oil can affect protection and may affect warranty coverage.
- Think about your driving. Synthetic may be a better fit for towing, extreme temperatures, and heavy stop-and-go use.
- Consider age. Some older or low-demand engines run fine on conventional oil.
Synthetic is not always necessary, but it is not just a marketing upsell either. For an engine that calls for it, it matters.
High-mileage engines
Many brands sell high-mileage oils, available in conventional, blend, and synthetic versions, which include additives intended to help condition seals. A synthetic blend or high-mileage oil can be a reasonable choice for an older engine if it meets the viscosity and specification your manual lists. If your engine already leaks or burns oil, ask a mechanic about the best approach.
Can you switch between types?
Generally, yes. You can usually move from conventional to synthetic or back, and mixing types in a top-off will not normally cause immediate harm. The main rules are to use the correct viscosity and specification. If you switch from synthetic to conventional, you should return to the shorter conventional interval. If an older engine develops leaks after switching to synthetic, have it checked.