Depends on weather conditions and use.
In very warm weather, a heavier fork oil may be needed, just as with engine oil.
In cold weather, a lighter fork oil may be needed, again just like the engine.
Plus track/racing use may require different oil than street riding.
It also depends of they type of valving installed in the forks.
On average, it can vary from 10, 7, to 5 weight oil for cartridge forks (which your bike has).
Most forks come with 10 wt oil from the factory.
Although the correct measurement is in centistokes (cSt) which is a measurement of viscosity as viscosity can very between oils of the same weight.