To try and clarify viscosity. a 5W30 oil is 5W viscosity at 20C, it is not 30W viscosity at all.
However, when hot, as is an engine and possibly the oil is 80C, the 5W thins, and while a 5W start viscosity will become extremely thin and runny, ie water, the 30W aspect of the oil means, because of it's chemical and additive properties, it does not become thinner than a normal 30W would be at that hotter temp. Therefore, while hot the system it is used in is running on what a 30W oil would be providing at that temp. A multigrade concept. In a gearbox you have to maintain a film of oil between the gearteeth faces and they are under considerable pressure for their face area. A synthetic oil has to have both cushioning and shearing ability to work for the teeth. Starting with too thin an oil, and then used when things get hot isn't going to protect the gearteeth as well. It is all a compromise and the sliding scale of oil applications in vehicle manuals reflect that.
Many years ago a mate of mine bought a 60 series cruiser from a telecom fleet. it had very noisey gearbox bearings, horrible noise. He could not afford a rebuild at that time so added Nulon/Teflon/PTFE to the box. Too much for normal use, but the gearbox did another 80,000km more before it was fixed.