Needless to say, you should always use the correct grade oil in your gearbox. Total manufactures it for Citroën/Peugeot with a grade of 75W. This is sometimes referred to as 'motorcycle gearbox oil'. Substitutes, ATF for instance, or any gearbox oil with incorrect grade, will leak from the top of the box, through the plastic tube where the gearbox lever enters the housing. Even with the correct oil it will leak slightly (it seems that the gearbox was not actually designed to operate in the slanted position it is fitted in).