4100 fire truck You know when I was young filling the differential I would fill it and have oil running out when I put the plug in !! I don't think that is such a good plan. It is OK if that oil level cold is 1/2 inch below the hole . This keeps from asking so much of the seals and when you get not level oil will travel both ways and fill the hubs. Of course you know to keep the vent open !! Better late than never !!