I've been running Losi 1/8th scales for years and the diffs don't leak on me to the extent I have fluid coming out anywhere. I'm sure some fluid is coming out because when I rebuild diffs they are never as full as when I assemble them, but not enough leakage that fluid is visble on the outside.

It is possible you might have a bad case or gasket or seals, but it is also possible you are just not building them correctly, and that is hard for someone to diagnose over the internet.

For the gasket, you can try a trick people sometimes used on SCTE diffs: Get some of the blue silicone gasket sealer from a auto store, "dilute" is with a mineral solvent (I used lighter fluid) and paint the gasket on both sides. When it dries, you will have it coated with additional sealant the should help seal the diff.

If the problem is the outdrives, you might consider using Associated RC8B3 outdrives number 81008. I haven't assembled a Losi diff with these yet but the measurements I took so far seem to indicate they will interchange with Losi parts. But these outdrives have room for a second O-ring (where the Losi's only have a groove) to seal the diff. That might work.

BTW, the reason I have the associated outdrives to begin with is I wanted a center diff with a larger plastic spur gear than Losi makes. Associated makes a 48. It looks like the RC8 diff will fit a Losi though I haven't tried it yet so I am not sure. Of course the 3.0 doesn't really have room for the larger center diff gear than Losi makes - I was wanting it for a 2.0.