When I did my Commando fork refurb first time this last winter, I used locally available Lucas fork oil 20wt. Using a large syringe (heroine addiction has side benefits) for metering in the spec amount, nary a drop lost. When I did this operation on my modern Bonneville there was no stated volume to add but instead a careful measurement of distance from top of tube. This meant the tubes needed to be held vertically off bike to fill. Clever folks buy a $30 steel pipe with a clamp to affix to tube and then draw out overfill oil to the correct level. I made one from a plastic straw and duct tape for less than $0.50.