I wouldn't ever use a GL5 oil in a french gearbox box.
Plenty of people are using the Fuchs Sintofluid (75w-80, GL5) in Clio, Megane, Peugeot and Citroen gearboxes without any issues at all. Technically, a GL5 oil should be backwards compatible with a GL4 application, but people often have concerns due to brass synchros. Quality GL5 oils are made with the same additives as a GL4 oil, but in higher quantities than in a GL4. Less reputable oil companies use different additives that attack yellow metals in GL5 oils, making the oil unsuitable for GL4 applications.
Cheers
Tim