gearbox oil leak

[datsunnyit]

This is a generic picture of 5 gear 240z gearbox.
What is a piece of white plastic stuff for?
Vent?
How do you know if it works properly?

Oil comes out where the transmission shaft is inserted.
Oil seal changed
But still loses oil
In the perimeter of the new oil seal it is necessary to seal with silicone?

thanks

https://www.google.it/search?q=240z...#imgdii=tx0zWHo3Vp-p7M:&imgrc=2PEfRwt5338SsM:

 

[Nigel Brook]

Should not be necessary to silicone the outside of the seal. Do you know where the box is leaking, front or back?

 

[datsunnyit]

back where is insert the propeller shaft.
I changed the seal but leak again

 

[Nigel Brook]

Did you check to see that the end of the tailshaft (the round end where the propshaft goes in) is not damaged ?