Cadillac 2008 XLR Owner's Manual page 338

What Kind of Engine Oil to Use
6-18
Look for three things:
•
GM4718M
Your vehicle's engine requires a special oil
meeting GM Standard GM4718M. Oils meeting
this standard may be identified as synthetic.
However, not all synthetic oils will meet this
GM standard. Look for and use only an oil that
meets GM Standard GM4718M.
Notice: If you use oils that do not have the
GM4718M Standard designation, you can cause
engine damage not covered by your warranty.
•
SAE 5W-30
As shown in the viscosity chart, SAE 5W-30 is best
for your vehicle.
These numbers on an oil container show its
viscosity, or thickness. Do not use other viscosity
oils such as SAE 20W-50.
•
American Petroleum Institute (API) starburst symbol