You should normally just fill it to the level of the fork marking. There is a small marking on the inside of the fork tube that shows you the level. Or look in owners manual for exact amount.
For most bikes, about 1 liter of fork oil of the correct weight is more than enough. The weight that you should use will depend on the type of bike that you have.
after tightening the screw only this fork oil has to be used to give the longevity of the nut without getting rusted. this allow the nut to get fixed easily and tightly in the screw so that it can move freely. the fork oil can be but as if it does not fall down in the earth.
