what to use to seal differential cover after changing the fluid?

I'm planning on changing out the fluid in both the front and rear differentials. The manual doesn't show a drain plug so it looks like I'm going to have to remove the cover to drain the old fluid. Manul call for a silicone sealer on the cover prior to reassembling. What product do you use? pematex formagasket, RTV or something else?

Thanks Steve