Before you do this, you should clean around the fill and drain plus with carb cleaner real good. Get all of the grime, sand, grit, etc. away from openings so you do not get any in the differential housing. Clean both the plugs real good before you re-install them. The fastest way to ruin a good differential is to introduce a small amount of sand or grit.
As far as fluid most people use the manual recommend gear oil, these diffs are pretty robust (unlike the transmission) and failures are pretty rare, so normal gear oil is probably fine. Some people prefer to put in synthetic gear oil, it lubricates better, lasts longer and may increase economy very slightly because of the lower friction. It costs a lot more, so I personally doubt the extra cost is worth any savings (I doubt you could even measure the improved MPG enough to matter).
'84 Tercel 4wd (current project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'86 CRX (daughter's daily driver)
'01 Suzuki Esteem (other daughter's daily driver)