As Chris said choice of diff and oil are down to personal preference. Have run a gear diff with oil in range of 1000-2000wt on carpet for last couple of years, have stuck with ball diff for all other surfaces which seems to be a good starting point.
Remember due to how a gear diff works (squeezing oil between gear teeth) the design and tolerance varies greatly between manufacturers and so its not really sensible to compare oil rates used in different cars.
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