Absolutely no problem at all in any vehicle, new or old, switching from one to another and back again.
If the car is going to sit around not being driven, try to use ethanol free. If you're cycling through the fuel pretty regularly, it doesn't make any real difference if you use ethanol laced fuel.
All of my cars get driven regularly and all of them use ethanol fuel. It's about all you can find here in Atlanta.
Non-ethanol fuel won't hurt new cars, either. Many places in America - mostly rural - don't require ethanol to be added to the fuel.
