Larry,
The oil is checked by looking down the vent hole on the top of the axle housing. The oil level should just cover the axle shaft. I found this out from a White FB 31 operators manual. White suggests using HiTran oil same as the transmission, but I know when I drained the oil in my TL2100F 4x4 (to replace a leaking spindle seal) it was 80w90 not HiTran, so I put 80w90 back in it.