The first thing I would check,
Would be,
Those bolts holding the pinion in the housing.
They may be loose causing it to seep through the shims.
Also about the type of oil,
I would pull the cover to see if by a slim chance that
It may have a posi-traction unit in it.
If not there is a few choices.
Food for thought: found on this site in the link below.
Link:
https://lugnut4x4.com/axles/best-14-...%20fill%20hole.
Why Use 75W-140 Gear Oil?
75W-140 gear oil is a good option for a wide range of temperatures. Unlike 80W-90 or 75W-90, 75W-140 offers protection at both colder temps (75) AND fairly high temps (140). While gear oil shouldn’t spend a lot of time at very high temperatures, it’s good to have protection in those scenarios. Likewise, you want protection at the lowest temperature possible for those few minutes when everything is still cold after start-up.
The downside to using 75W-140 gear oil is the cost. It’s about twice as expensive as conventional 80W-90, and maybe 30% more than conventional 85W-140 gear oil.
Still, the absolute cost difference is typically less than $20. For $20, it’s kind of a no-brainer.
.