the factory manual says 20 ft-lbs.
make sure the differential cover is not warped, and that bolts are tightened in a gradual cris-cross pattern (like wheels).
I use a new GM gasket every time the cover is removed, and also apply a non-hardening gasket sealant, such as Permatex #2 or Hylomar.
For me, this has performed on 2 vehicles -- no leaks whatsoever.
