have you fill it in correct volume ?
you should fill it until the oil is above the bevel shaft a bit.
too much oil will cause your diff to leak from its oil overpresure.
if that thing still happen then you should double your gasket seal (i read this in www.sportwerksrc.com --> tame that mayhem)

but it never happen to me anyway. and they must not evaporate or escape from anywhere coz it could damage the bevel teeth.
i rebuild my diff after 3 weeks of praticing and once after racing.

if that still happen then try greasing your diff instead of silicon oil