Best but is to read the user manual which states 89 octane
The 3.6 engine can take regular (89 octane)
Either engine on the 05 CTS runs on 87 Octane gas. The 05 CTS-V runs on premium only.
Premium
Cadillac recommends premium fuel in this engine.
If you are refering to regular gas, it's not recommended. Some people use it as well as myself due to the fact of gas prices. What I do is add a gas octane boost to compansate for the lower octane fuel.
The 2004 Cadillac CTS runs on regular unleaded.
The 2005 Cadillac CTS runs on regular unleaded.
The 2008 Cadillac CTS runs on regular unleaded.
The 2006 Cadillac CTS runs on regular unleaded.
The 2010 Cadillac CTS runs on regular unleaded.
The 2003 Cadillac CTS runs on regular unleaded.