can I switch back to regular oil after using synthetic oil
Yes. Feel free to switch from either one to the other. Just be sure to use the correct viscosity of whatever type of oil you buy.
www.carbibles.com has a good primer on oils. I recommend a visit.
There is nothing about synthetic that would prevent you from switching to non-synthetic oil.
There is something that might mean you should not switch however. What does the owner's manual have to say about what oil to use? If it specifies synthetic, then you would be a fool not to follow their instructions.