There is a rubber o-ring that goes between the t-case adapter and thransmission housing then a paper gasket between the adapter and t-case. And the adapter is not like the 205 adapter, there is no seal inside of it. The front of the 208 should be sealed but fluid will leak out of the adapter housing if you dont seal it up.
RTV will work on the t-case side if you clean everything up real good but I prefer to just get the gasket, Ive had better luck. I never use rtv on the tranny side, always used the o-ring.