Yes, these hubs are "splash" lubricated. They call for oil which circulates like the action of front loading washing machines.
Gear oil, or any non-detergent machine (not motor) oils is fine.
The advice to avoid motor oil is because som we IGH hubs use permanent bonded lubes which can react with additives on motor oils. If you know its not an issue, then 20-30 weight motor oil is fine.
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