I'm not sure about the oil check plug. My b 7100 had a dip stick on it.
As far as the oil, I never had a book for my tractor but the dealer told me it required 85w140 gear oil. I have no idea if that was correct but I ran it for 16 yrs (over 6000 hrs) without any trouble from my diff. If I remember correctly mine held about 3 gallons or so.
Maybe some of the other guys will be able to help you with this better than I have.