75W80 is the best oil for our pug ML5T, BE3/5 and BE4/5 gearboxes
i know volvo specify and interval of 6 years or 60,000 miles under arduous conditions, i.e. towing or taxi drivinmg etc
pug dont however
i changed mine at 105,000 when i did the clutch and it didnt come out that bad, the main reasons for changing gear oil are, the first, build up of metal particles, the magnetic drain and filling plugs can only hold so much, the rest then floats in the oil, and second is moisture absorbtion, unlike an engine which can burn off the moisture through the PCV system, there is nowhere for it to go in a gearbox so it ends up being compressed into the oil making it less viscous and causing added wear and the risk of corrosion too