Should be 90w gear oil. Let me look.
Well a picture I have doesn't show much. Basically you fill it like a diff, remove a plug, add oil until it runs out, replace plug. On my 2500 it's .18qts, not much. Manual does show to use UDT or SAE 80, 90w oil. I would stick with 90w, heavier incase the seal is leaking a bit.
Rob