dif slip additive a must
That hex you are talking about is simply your 3/8" socket wrench tip. Use an extended socket shaft and it will make it easier to work with. I experimented with the white plumber's tape (teflon) and wrapped the hex screw threads for better sealing. The rear end stays cleaner and won't weep gear oil out. Be careful not to over tighten the hex too much!
You have mini-clutch packs inside the Locking Differential Unit. This takes special oils. There is no advantage to move away from the recommended oils provided by the manufacture. In fact, the factory oils are specially formulated for the Saginaw, Borg-Waner and Dana rear ends. Why take the chance?
The problem using the wrong oil or no additive at all to the LDU clutch plates, will gum up and not release the clutches when turning a corner. This causes clutch chatter and accelerates tire wear.
I strongly recommend using 80w-90 GL-5 (GM Part No. 1052271) and adding the limited-slip diff. additive (GM Part No. 1052358). Oil capacity for your rear end is approx 4.5 pints, +/- 0.2 pints.