Synthetic oil may last anywhere from 5,000 to 7,000 miles before it needs to be changed, although some brands may last as long as 10,000, 15,000 or even 20,000 miles. Change interval length also depends on factors such as road conditions and trip mileage.
Synthetic oil in vehicles driven in dirty and dusty conditions must be changed more frequently than those that are cleaner. Cars that make longer trips also need more frequent oil changes, since oil doesn't get hot enough to boil off condensation during shorter trips. Synthetic oil is cleaner and better for car engines than conventional motor oil, providing less resistance and squeezing out more power.