Although synthetic oils are perfect for the engine, don't use them in the gearbox. Citroën gearboxes (the synchro rings, actually) seem to rely on mineral oil which is thicker than its synthetic counterpart. I committed the same mistake: trying to be smart and go for the best, I chose Mobil Mobilube 1 synthetic. Although it worked, changing gears became harder. A couple of days later I went back to the originally specified oil (Total BV 75 or EP 80 are mentioned in my instruction manual, Castrol also manufactures the second) and it's as smooth as you can get.