Hi I have a Prima 50 hp in my Beneteau You would need to use "transmission fluid in the gearbox [hurth] only !!! and a good quality diesel oil in the engine itself Transmission fluid is for gearboxes only !!! In my gearbox I use the normal "auto transmission fluid" [ATF] This gearbox is a "splash type box" and only needs to be filled to the indicated line on the "dipstick"