According to my repair manual, the front and rear differential uses hypoid gear oil, SAE 80W. This is from a 1995-2000 Honda Fourtrax 300 4X4 repair manual. It is the Honda brand manual. It says to use Honda GN4 motor oil (10W-40) in the front gear case(transfer case), but I think a lot of people use the 80W gear oil in it and it seems work fine. Welcome to the forums.