OBH wrote: ...I think once the correct viscosity has been ascertained there will be no need to use the Shimano oil. It's not like this is a high stress performance part, I'm only plodding through the local wood at 15km/h.
Gearboxes transmit torque at a certain speed. Multiply the two and you get the power transmitted.
Torque rating is the most important thing with gearboxes; they generally operate happily over a range of (higher) speeds so power transmission can go up simply with speed (at fixed torque) and the gearbox doesn't care. However double the torque at half the speed and suddenly you are in big trouble, even though the power is the same.
Low speed use is doubly bad because the bearings only reach their full load-bearing capacity above a certain speed, operating in EHD or hydrodynamic mode according to their type. The threshold speed for this will vary with lubricant type, which is why low viscosity lubricants are not a good idea in hub gears; to a good approximation the thicker the better unless it causes operational difficulties like poor shifting.
Below the threshold speed, the bearings and gear teeth contacts will be operating in a mode whereby some metal-to-metal contact and wear will be expected. Cleanliness, high viscosity, EP additives and solid lubricants are the only defences against wear in the long term.
Very few hub gears have shown a tendency for the gear teeth and bearings to wear exceptionally fast, so maybe the main benefit of better lubricants is simply a much longer service life.
Pretty much any clean
oil is going to be better than a porridge of wear or running-in debris.