Chevrolet Monte 2003 Owner's Manual page 267

As in the chart shown previously, if you have the
3400 V6 engine, SAE 5W-30 is the only viscosity grade
recommended for your vehicle. You should look for
and use only oils which have the API Starburst symbol
and which are also identified as SAE 5W-30. If you
cannot find such SAE 5W-30 oils, you can use an
SAE 10W-30 oil which has the API Starburst symbol, if
it's going to be 0°F (−18°C) or above. Do not use
other viscosity grade oils, such as SAE 10W-40 or
SAE 20W-50 under any conditions.
As shown in the following chart, if you have the
3800 V6 engine, SAE 10W-30 is the viscosity grade
recommended for your vehicle. However, you can use
SAE 5W-30 if it's going to be colder than 60°F
(16°C) before your next oil change. When it's very cold,
colder than 0°F (−18°C), you should use an SAE 5W-30
oil. These numbers on an oil container show its
viscosity, or thickness. Do not use other viscosity grade
oils, such as SAE 10W-40 or SAE 20W-50 under any
conditions.
3800 (Code K) V6 Engine
5-19