Cadillac 2005 XLR Owner's Manual page 323

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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.
You should look for this on the oil container, and use
only those oils that are identified as meeting GM
Standard GM4718M and have the starburst symbol on
the front of the oil container.
Your vehicle's engine is filled at the factory with a
®
Mobil 1 synthetic oil, which meets all requirements for
your vehicle.
Oils meeting these
requirements should also
have the starburst
symbol on the container.
This symbol indicates
that the oil has been
certified by the American
Petroleum Institute (API).
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