Best but is to read the user manual which states 89 octane
The 3.6 engine can take regular (89 octane)
Premium
Cadillac recommends premium fuel in this engine.
Either engine on the 05 CTS runs on 87 Octane gas. The 05 CTS-V runs on premium only.
The 2009 Cadillac CTS runs on regular unleaded.
The 2010 Cadillac CTS runs on regular unleaded.
The 2003 Cadillac CTS runs on regular unleaded.
The 2005 Cadillac CTS runs on regular unleaded.
The 2008 Cadillac CTS runs on regular unleaded.
The 2013 Cadillac CTS runs on regular unleaded.
The 2006 Cadillac CTS runs on regular unleaded.
The 2012 Cadillac CTS runs on regular unleaded.