Cadillac 2011 STS Owner's Manual page 369

SAE 5W-30 is the best viscosity grade for the vehicle.
Do not use other viscosity oils such as SAE 10W 30,
10W 40, or 20W-50.
Cold Temperature Operation: In an area of extreme
cold, where the temperature falls below 29°C ( 20°F),
an SAE 0W-30 oil should be used. An oil of this
viscosity grade will provide easier cold starting for the
engine at extremely low temperatures. When selecting
an oil of the appropriate viscosity grade, be sure to
always select an oil that meets the required
specification, dexos . See Specification for more
information.
Do not add anything to the oil. The recommended oils
with the dexos specification and displaying the
dexos certification mark are all that is needed for
good performance and engine protection.
Engine oil system flushes are not recommended and
could cause engine damage not covered by the vehicle
warranty.
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