Tech tip: the best way to achieve a completely leak free and consistently performing XRAY T4 differential. Consider this as a supplement to the instruction manual:
1. Don't use the o-ring for sealing. It swells up when in contact with oil, and CAN cause issues with leaking (it doesn't always). So I recommend using only the gasket.
2. Use longer screws for the diff case (8mm, instead of the supplied 6mm). I recommend the XRAY part number 903258. Alternatively, the titanium ones from HIRO SEIKO, part number 69891. The longer screw will minimize any issues with stripped threads in the case, and you can comfortably crank down on the screws to completely seal the diff.
3. Ream small holes from the back of the diff case, to let any excess oil escape. This will prevent any problems with stripping of the threads as well.