Pontiac 1999 Firebird Owner's Manual page 263

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As shown in the chart, if you have the 5.7L V8 engine,
-
SAE 5W 30 is best for your vehicle. However, you can
30 if it's going to be 0 _ F ( 18 _ C) or
- -
use SAE 10W
above. These numbers on an oil container show its
viscosity, or thickness. Do not use other viscosity oils,
-
such as SAE 20W 50.
5.7L V8 Engine
6-15