have not been on the old allis chalmers CA (next model up from B if memory serves). i use a universal hyd fluid, dark yellow / gold strips on the label, in a black container. it is not the cheapest, but about average price compared to all the other brands out there.
if ya have a tractor supply, farm and fleet, farmking, or other farm store nearby. goto there 2 to 5 gallon bucket section for oil. and just read the labels, most of them, will list a brand/model number. of what the oil was designed for. (if ya have hard time reading small print, bring your glasses)
not sure were the fill up on rear end on a B is. but spent some time checking for a better funnel that did not fall out as easily compare to other generic funnels.
old tractors, tend to have leaks, so be prepared to check your fluids often, and fill as needed. this means checking oil every couple hours during use. till ya get idea how tractor will behave.