What is the best gas to put in a Cadillac STS 1999?

The best gas for a 1999 Cadillac STS is premium unleaded fuel with an octane rating of 91 or higher. Using higher octane fuel helps optimize engine performance and efficiency, as the STS is designed to run on premium gas. While regular unleaded may be used in a pinch, it is not recommended for long-term use due to potential knocking and reduced performance. Always consult the owner's manual for specific fuel recommendations.