It's physically possible to do so and the engine will run, but you shouldn't make a practice of it. The engine will run roughly and inefficiently. It will get worse over time.
You can but you gonna have problems after 300k and performence wise you would feel like driving a toyota. If you put premimum theres less chances of repairs its a small difference
if its regular gas
Yes.
Regular.
Yes
Never put gas in a diesel car. And never put diesel in a gas car. An engine is built to handle one and not the other.
Regular unleaded.
Regular Unleaded.
Regular unleaded
" regular " unleaded , 87 octane
You will risk doing damage to the catalytic converter(s).
In order to preserve the motor in your CTS you should put only premium gas in it.