Well, I have to say that sealing is more of a prep and cleanliness issue.
I have had the best luck with a THIN coat of clear silicone sealer on the gaskets, after wiping the surfaces down with non-chlorinated brake cleaner like NAPA p/n 4800 or 4810. Then allow it to dry before adding oil.
Specifically in dealing with the cluster shaft, I spray the case and shaft with cleaner, let dry and then apply the silicone.
Think clean, clean...clean. This applies to all devices that are to be sealed. But is my input on transmissions.
If you dont plan doing rebuilds too often and really want it sealed, I can also recommend a sealer or two that will not leak.
(Yeah, get this) Toyota Seal Packing p/n 00295-00102.
A sealer called Wacker. Its an OE drivetrain sealer that DANA Speicer uses to seal their transmissions, transfer cases and differentials. It is orange in color....