The reason the 5th gear fails is lack of lubrication to the 5th gear, the oil having to pass through the 4 speed casing bearing. The solution when fitting a new 5th gear is to drill holes through the bottom of the casing to equalise the oil level in the 4 speed casing and the 5th gear extension casting.
Over filling a little if unmodified is unlikely to improve lubrication as it still has to pass through the bearing. Gearbox molyslip would help.
Do not over work 5th gear. Change down early and keep the revs up.