A word of
'CAUTION' regarding transmission/transfer case gear oil:
'DO NOT USE Hypoy/EP 90 based products! They will damage your gears!'
The two most important things here are these: 1. Replace the lube with a straight mineral oil…. 90-grade. 2. Under NO circumstances be tempted or persuaded to use Hypoy/EP 90, unless you don't mind rebuilding boxes every couple of months.
Here in England we can obtain straight 90s mineral oil in the form of Castrol ST-90..there are others, too, such as Morris Lubricants' AG-90. You will certainly have an equivalent to this in the USA and Europe. Make doubly sure it is NOT hypoy/EP. If you use straight 90s-grade oil in the transmission-transfer, it makes sense to use the same oil in the differentials, too. That way, there's no chance of putting Hypoy-EP in the gearbox by mistake.
Now for the history lesson…..the Military originally used GO (Gear Oil) and the civilian equivalent gear oil was designated “GL-1” which was a non-Hypoy/EP gear oil which did not affect the phosphor-bronze components in the transmission. After GL-1, came GL-2, -3, -4, and then 5. Each GL number represented a jump forward in technology. (Read, less wear, better protection, etc) GL5 when it first came out had a problem with the sulphur eating up yellow metals - brass - usually used for synchro rings. This occurred only at high temperatures as the oil started to break down. The industry recognized that and the MT-1 designation was added along with additional specification requirements for GL-5. So basically a gear oil with the designation “GL-5 with MT-1” should be safe for yellow metals according to current standards. Texaco, Valvoline, Mobil all make good gear lube and it should clearly state “GL5 - MT1”.
'TWO drain plugs' but only
'ONE fill plug'. Everyone thinks there must be another fill plug, so they look higher up the gearbox and see a recessed plug which looks just like a filler plug.
'IT ISN'T !' It is the reverse-idler-arm retainer and if you undo it, the arm will drop down inside the gearbox.
The correct (and only) fill plug is low down
(much lower than you would expect) on the side of the box.
Because of errors caused by the similarity of the fill plug and the idler retaining bolt, the Mil tried two modifications. The first one was to add a lock-washer to the idler bolt. The tabs of the lockwasher were then bent around the bolt so that it could not easily be removed.
This wasn't the success that it was intended to be.
A further modification involved the removal of the idler arm bolt and its replacement with a socket-headed plug, which needed an allen key to remove. Whatever type of plug is in there, remember to keep your hands off it unless you are actually removing the reverse idler arm.
The easiest way to refill the boxes is to undo the fill plug, but pour the oil down the gearshift housing after temporarily removing the gearlever…..it is much easier than lying under the truck trying to pump oil up into the fill hole.
The capacity is 5.5 US pints….2.6 liters.
Drain the transmission and differentials after a run, when the oil is hot.
NB When removing the gearlever, always make sure that the lever is in NEUTRAL position. If you do not, you may find that re-inserting the lever can be very difficult.
'Correct Oil for differentials'
See comments above.