6 litres should be fine providing that your oil cooler does drain back into the sump on its own. If not, then you may over fill your sump. Not that 1 litre will make too much of a difference.
Id say for the first time changing oil with the cooler, put 5 litres in, start the car, let the car idle for 10 mins or so, turn car off then check the oil level a couple of mins later. If its a little low then your cooler is holding oil and wont drain back. Then you can add another litre in to top it up. In which case all you have to do is put 5 litres in the next time if the cooler wont drain on its own. So it'll be just like a normal oil change.