Only Red antifreeze type is suitable and you’ll need 11.3 liters of it. Otherwise C6 made in 2011 won’t serve you long enough. It’s important to provide cooling for Citroen’s engine with 2.2 liters volume. For sufficient use of Citroen C6 you need to use suitable antifreeze. In 2009 the company produced vehicles with 2.9 engine having code of ES9J4S.
A lot of ES9J4S motors are used long in Citroen cars but only with good service. So better use Citroen’s advice for servicing 2008 C6. Produced in 2012 the 3.0 engine in C6 model will serve longer by this. If you choose other than Red coolant or use less than 11.3 liters, you can’t expect normal work.