GMC 1994 Sonoma Owner's Manual page 242

I
NOTICE:
If you use an improper coolant mix, your engine could overheat
and be badly damaged. The repair cost wouldn't be covered by
your warranty.
Too
much water in the mix can freeze and crack
the engine, radiator, heater core and other parts.
Some conditions, such as air trapped in the cooling system, can affect the
coolant level in the radiator. Check the coolant level when the engine is cold
and follow the steps under "Adding Coolant'' for the proper way to add
coolant.
If you have to add coolant more than four times a year, have your dealer
check your cooling system.
I
NOTICE:
If you use the proper coolant, you don't have to add extra
inhibitors or additives which claim to improve the system. These
can be harmful.
Adding Coorant
To Check Coolant
When your engine is cold, the coolant level should be at "ADD," or a little
higher. When your engine is warm, the level should be up to "FULL HOT,"
or a little higher.
To Add Coolant