the citroen oil is called "LHM + (LHM plus) its green and its cheaper then Royal blood, its also a mineral oil like royal blood, but you must make sure you get all the royal blood (blue in colour) out before bleeding with LHM + as it doesnt mix (like water and oil doesnt mix, i work in the motor trade so get it cheap, but i allways use Royal blood as its whats recommended buy Magura, (even though car manufacturers change the 'recomended' brand of oil every year or 2 (only due to who there on stop with at the time of manufacture) you could argue you could use any mineral oil, which is partly true, as most people use any fluid in a Magura, water, anti-freeze, corn flour etc etc)
its a pre-filled system so aslong as it doesnt compress any fluid is fine
What dragged me to this topic is because my mates having this problem, we bled it 4 times with royal blood and slave which only has the cross over attached and you bleed it from (not the cross over and the main realy long hose going to the lever) sticks, but after a few pumps it free's itself and works fine, until you leave it a few minutes and it sticks again which doesnt leave much confidence, whilst on a ledge and you need the brake to hold whilst about to gap......
we cant work out why, unless the slave needs replacing??
Maguras are EPIC when they work, but a PAIN IN THE A*** when they dont
P.S. mines fine though bites when the lever is pulled in about 3mm and sounds LOUD AS F**K!!!! (just the way we like it)