Can you use unleaded gasoline instead of premium unleaded in a cadillac?
Yes. Though you didn't mention what year model. Usually it's stamped on the fuel door of what is recommended for your car. Typically if it is a low compression, 8 to 9:1 low to mid grade unleaded is all you need and high octane or premium isn't necessary.
Premium is necessary for high compression engines, typically in BMWs and Mercedes, that are usually 10 to 11:1. And it is stamped on the fuel door. If anything less you risk bad detonation or "pinging". Which would cause left deposits and engine deterioration.
Needless to say you could run premium, if it's not recommended, though it would be a waste of money at the pump.
Yes you can use premium gasoline in unleaded because premium gasoline is also unleaded. The only reason it is called premium gasoline is because it has a higher octance then regular unleaded. Regular unleaded has an octane rating of 87 and premium has an octane rating of 92. Premium gasoline burns slower then regualr gas because it has a higher octane rating.