Cadillac 2011 STS Owner's Manual page 386

DOT 3 brake fluid is recommended. DOT 4 brake fluid
is also compatible with the vehicle's brake system parts.
However, if DOT 4 fluid is used, it is recommended that
the brake hydraulic system be flushed and refilled with
new DOT 4 fluid at a regular maintenance service every
two years. See
Use new brake fluid from a sealed container only. See
Always clean the brake fluid reservoir cap and the area
around the cap before removing it. This helps keep dirt
from entering the reservoir.
With the wrong kind of fluid in the brake hydraulic
system, the brakes might not work well. This could
cause a crash. Always use the proper brake fluid.
6-36
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This vehicle has disc brakes. Disc brake pads have
built-in wear indicators that make a high-pitched
warning sound when the brake pads are worn and new
pads are needed. The sound can come and go or be
heard all the time the vehicle is moving, except when
applying the brake pedal firmly.