Amount/kind of lube in rear differential?

[Henry Claeys]

(1) I removed the top plug, from the rear differential, and had to insert
nearly my entire index finger before locating any lube. Am I correct in
assuming that the lube is low? (2) What type lube should I use to refill? (3)
There was a second plug, located lower, which I assume is the drain plug.
Should I drain the lube before adding more? Thanks. Henry@...
1979 FC 35, McAllen, Tx.