locomotive,
You should be adding up to the filler plug hole. Be sure to match the lube to the type axle joint you have. I've read 1/2 pint (not sure why it can't be stated 1 cup or 8 oz), BUT...
The Service Manuals (SM) state how much and when, in the LUBRICATION section.
The early SM, SM-1002-R6, says: Check every 1,000 mi, change each 10,000 mi. Amount- as required.
The later SM, SM-1046, says: Check every 2, 000 mi, change each 12,000 mi. Amount - as required.
A Service Manual is a GREAT source to have in your shop!