The supported answer is water
The original question came from a player who kept searching for coolant rather than driving. Several different replies say that they used water successfully. The name of the task led the original poster to expect a separate container.[1]
That evidence supports trying water for the Demo radiator task. It does not prove that a dedicated coolant item can never exist, or that every future version behaves identically. Use the item name and current interaction text rather than guessing from a bottle’s colour.
Check the transfer, not just the container
This is a checking sequence, not a claim that we have verified a universal fill point. The discussion does not document the radiator capacity, exact control or required volume. If your character drinks instead, stop and recheck the target and prompt before using more water.
Do not use “the engine started” as the whole check
One participant said they drove without coolant, while another warned about eventual engine damage. That exchange does not measure a safe driving time.[1] Separately, the developer’s June update says engine heating and radiator cooling were reworked so overheating occurs under real conditions.[2]
Check the task and vehicle feedback rather than assuming an engine that starts is fully prepared. If the coolant task is complete but ignition fails, use the no-start diagnosis.