There actually are some Service Bulletins on this
Hi Lauren
There actually are some Service Bulletins on this issue of what height to fill different transfer cases in different application situations.
They all boil down to this the fill to the bottom of the filler hole you have marked Middle in your photo.
The Service Information Bulletins contain a wealth of information and updates to what was in the manuals. In general the design of the transfer cases and placement of the fill plugs started off having a much larger capacity than what was needed and because of the additional height of gear lube in the boxes a larger leak problem. In some cases the bulletins even recommend drilling and taping for a new lower filler point. Exceptions to the lower level of gear lube were when the winch was being used extensively.
In particular Bulletin B-55 dated May of 1943
Now as to the filler point on the front of the case on the "declutch" this really only needs (in my experience) to be used if the transfer case has been disassembled, hence really drained of all lube. On some transfer cases if you remove that top front plug when the transfer case is hot the lube will actually run out because the filler plug on the back is higher.
Obviously it is how much is it leaking, if the transfer case is over filled the leakage gets worse. I consider that things are normal when I can top off the transfer case in the spring and still touch lube with a crocked finger at the end of the summer. Watch out though transfer case and lube maybe hot, like 140F after several hours of road driving. If you are fortune enough to be able to park the truck in the same spot on a hard surface you can gauge how much it is dripping.
Tracking the temperature watching the drip marks is my way of keep an eye on things. I've now got records for all three trucks on wheel hubs, transmission, transfer case etc. A quick walk around the truck with an IR gun reading the temps when ever I drive the truck and hour or more and you start to spot stuff.
If your truck gets to the point where it is marking its territory with large puddles of oil or lube then you really have to start watching fluid levels very closely.
Hope this helps.
Cheers Phil
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