I have a 2004 Mazda 6. How often do I need to change synthetic oil?
Change it at the interval listed in the owner’s manual.
Does the time between changes differ if you use synthetic oil?
Nope, unless you want to do some oil analysis (compainies will do it) to come up with intervals particular to your driving habits.
No. Despite claims to the contrary, the interval does not change.
Yes and no.
For most of us it is NO. There is no automatic extension of oil change intervals granted by using synthetic. There are just too many factors to give a simple yes to that question.
What might be OK for one engine may not be OK for another. Oil does a lot of things in your car and when it can no longer do them all, it needs to be replaced. Without careful monitoring of the oil you will not know what is safe. The only exception to that rule would be if a manufacture did the test and made the recommendation. I don’t know if any manufacturer has done that.
All else being equal synthetic is going to provide superior protection for your engine. It is safe to use in all engines today (the early synthetics had problems with some engines) Make life simple, just do what it says in the owner's manual.