Ah yeah, everything is metric on japanese bikes. Your oil leak is probably not due to the oil level I'm sorry to say. That sure would be the easiest to fix but it's more likely to be a leaking seal, washer, or gasket. Once you have your oil level set get the outside of the engine real clean with some degreaser or a strong grease cutting soap. The wind blows the oil all kinds of places, even backwards "up" the direction of the wind, so you have to get all that off then wait and see where the new oil is coming from.
The most common places are under the left side engine cover. There are oils seals around the sprocket shaft, the clutch pushrod, and the shift shaft that can all leak as they age. Sometimes you get oil coming from the copper sealing washers on the underside of the engine too, and up top the valve tappet adjuster covers sometimes leak as well as the seals behind the points and advance housing.