The volume of the tank on your 308 is 17L.
When the capacity of your tank reaches a driving range of less than 1500 miles and up to 375 miles,of mileage, an audible warning signal sounds and the UREA warning light flashes displaying the message “Top up emissions additive: Starting prevented in 400 miles” every 200 miles driven. Between 0 and 375 miles, the indicator light is lit and the UREA warning light flashes, accompanied by a tone and the message “Top up AdBlue®: engine start impossible in 375 miles” every 30 seconds. The remaining distance is displayed between 250 miles and 0 miles at intervals of 30 miles. If your tank is empty, fill up with 3,8L to turn the warning light off.
When your tank level reaches 0 miles, you will not be able to restart the engine once it has stopped. If your engine is still running, you can drive to a service station. If the car does not restart immediately, wait for two minutes for the system to detect the AdBlue® top-up. If the AdBlue® tank is empty, turn the key in the ignition for about 15 seconds and then start the engine.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.