Where to fill front axle on 4 WD Foton 404

I have a 2006 Foton ft540a and the front axle on mine actually has 3 places on the front axle to have oil.You have one at each wheel then the center gear assembly. The ones at the wheels have a drain at the bottom of the wheel steering joint assembly with a fill level plug around 5 inches from the top. The one in the center has a drain in the bottom of the gear case and a fill plug on top of the axle housing .I was told to get a flashlite and shine it in the fill hole and fill till it covers the axle shaft. Mine has a site glass for a fill level for the axle but it is useless. Yours might have a dipstick which would be better.

just gear oil 85w90w. I think a gl-5 there should not be any yellow metal in there .but if you want to be safe use a gl-4  gear oil. What I know about that is some gl-5 oils use chlorine to formulate the oil and that is corrosive to copper brass and such. I have seen oils that are rated gl-5 and 4 so I assume that they are non-corrosive plus meet higher stress levels. anyone else on this? I am not a  oil guru.