Lexus GS 350 Owner's Manual page 605

604
7-2. Steps to take in an emergency
NOTICE
When adding engine coolant
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Add coolant slowly after the engine has cooled down sufficiently. Adding cool coolant
to a hot engine too quickly can cause damage to the engine.
To prevent damage to the cooling system
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Observe the following precautions:
Avoid contaminating the coolant with foreign matter (such as sand or dust etc.).
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Do not use any coolant additive.
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GS350_300_GS F_OM_OM30J17E_(GE)