GMC 1999 Sierra 1500 Pickup Owner's Manual page 302

Service & Appearance Care
HOT
WEATHER
LOOK
FOR THIS
LABEL
SAE 5W-30
SAE 1OW-30
PREFERRED
above 0°F
(-18°C)
IF NEITHER SAE 5W-30 NOR SAE 1OW-30
GRADE OILS ARE AVAILABLE, SAE 30
GRADE MAY BE USED AT TEMPERATURES
ABOVE 40 DEGREES F (4 DEGREES C).
DO NOT USE SAE 1OW-40, SAE 2OW-50 OR
ANY OTHER GRADE OIL NOT RECOMMENDED
PO347
SAE 1OW-30
Vehicles with Heavy Duty Emissions (8501 GVWR or more).
As shown in this chart, SAE 1OW-30 is best 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, below 0°F (-18"C), you
should use SAE 5W-30.
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