This should help.

 

After draining out the old oil from the primary, refill with fresh oil until the oil just touches the bottom of the clutch basket. The clutch basket is that big round hub that is inside the primary case and houses the clutch. I will take something around a quart. If too full by a smidgen it won't hurt anything but neutral can be a little harder to hit and it will not shift as well. So, refilling is not 'critical" but if you want it to shift like it is supposed to do refill until the oil reaches the bottom of the clutch basket. It is hard to see so get out your flashlight.