“I was told by the salesman to use synthetic oil; that it would decrease the number of miles necessary between oil changes.”
Hmmm…most people would consider that decreasing the number of miles between oil changes is a negative factor, so I’m not quite sure why this would be a selling point. Most salesmen would try to convince you that the more expensive synthetic oil allows you to increase the number of miles between oil changes.
And, then we have the service department that is undoubtedly trying to get the OP to change her oil more often than is really necessary.
The simple answer to this question is to utilize the maintenance schedule provided by the manufacturer. It is contained either in the Owner’s Manual, or in a separate booklet with an appropriate title. In that maintenance schedule, there is an elapsed time factor as well as a mileage factor (“every 6,000 miles or 6 months, whichever comes first” is an example of many car mfr’s oil change specifications) for most maintenance procedures. Unless the OP becomes aware of the car’s maintenance requirements and adheres to them, there is a very real chance of voiding the car’s warranty.
As to the salesman’s statement that you can’t switch from synthetic oil to conventional oil, that is also incorrect. Some folks were of this belief when synthetic oil was first introduced, but this notion was debunked many years ago.
Over the years, I have found almost all car sales people to be woefully ignorant about the features of the cars that they are selling, and to be absolutely NOT knowledgeable about car maintenance.
The OP really needs to begin reading that Owner’s Manual.