Synthetic oil is engineered from more uniform base oils, so it flows better when cold, resists breaking down at high temperatures, and keeps its protection longer than conventional oil. Conventional oil is refined from crude oil and costs less. Synthetic blends sit in between. Many modern engines, especially turbocharged and direct-injected ones, require synthetic oil that meets a specific standard; older engines can use either, as long as the oil meets the grade and specification in the owner’s manual.
Synthetic vs conventional oil
| Conventional | Synthetic blend | Full synthetic | |
|---|---|---|---|
| Cold flow | Good | Better | Strongest of the three |
| High-temperature stability | Good | Better | Highest |
| Change interval | Shorter | Moderate | Often longer, per manufacturer |
| Price | Lowest | Middle | Highest |
Synthetic oil for older cars
Synthetic oil does not cause leaks in older engines, but it can find existing weak seals because it flows more freely. If an older engine starts leaking after switching, the seals were already worn. Many high-mileage oils add seal conditioners for that reason.
How often to change oil
Follow your oil life monitor or the interval in your manual. Many modern cars on synthetic oil have intervals in the several-thousand to ten-thousand-mile range, while older engines on conventional oil often used shorter intervals. Use the severe-service schedule if you tow, make many short trips, idle a lot or drive in dust or extreme heat. Change the oil filter with every oil change.
Oil for diesels, boosted and performance engines
Diesels need oil that meets diesel specifications, and turbocharged and supercharged gas engines often require a specific synthetic. Boxer and high-revving engines may consume some oil, so check the level often; see FA20 problems and EJ25 problems.
Viscosity and specifications
The grade, such as 0W-20 or 5W-30, describes how the oil flows cold and hot. The specification on the bottle, such as an API, ILSAC or manufacturer approval, matters just as much. Use both as your manual specifies.
Is synthetic worth the cost?
For engines that require it, there is no choice. For older engines that allow conventional oil, synthetic costs more per change but protects better in extreme cold and heat and may allow longer intervals where the manufacturer permits. Drivers who tow, live in very cold or hot climates, or have turbocharged engines usually benefit most.
Signs of oil trouble
Low oil level, oil pressure warnings, milky oil or rising oil consumption need attention; see oil pressure light, milky engine oil, engine burning oil and oil leak.
More guides: oil pan gasket leak.
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