Hi.
About leaking diffs. and the big O-ring.
All new batch of diffs. are delivered with a new big O-ring. The new O-ring is made of silicone (same material as the x-ring in the diffs.) and is slightly bigger in the material than the old black one. I have no leaking at all with the new O-ring. Soon you can order the new O-ring as a spare part. Serpent told me, that they should be available this week. I don't know the order no. yet.
With the old black O-ring you can use Asso greenslime to prevent leaking as some other mentioned. It really helps. Also be sure to tighten the screws even not to flex the diff. cups.
When I assemble the diff. i sand all gears on the backside. Especially the two big metal gears, as I want to have slightly less pressure on the x-ring to let the outdrives run smoother. Then I use Tamiya blue damper grease on the x-ring, but you can also use Asso greenslime. As I wrote above, I have no problems with leaking.