Gilles,
         I have a similar problem with an 87 Westy.   It is a small leak that occurs when I am going up and down in the mountains.
I have not tried changing the seal as it is a small leak.  In my case I think if the seal does not fix the problem it is something about the transmission itself that will become more apparent with time.   Tranny has 230K and never had anything done on it.   I am thinking bearings are getting a little loose and things are moving around in away that causes the gear oil to kick around a bit more than usual?   Gary

--------------------------------------------
 Date:    Thu, 9 Jun 2016 15:28:31 -0400
 From:    Gilles H Turmel <gilles.turmel@SYMPATICO.CA>
 Subject: Tranny oil leak
 
 Hi,
 I have a 1989 Westy Syncro, oil is dripping from the reverse
 gear cover. =
 I=E2=80=99ve removed the oil, used Loctite 518 to make a new
 gasket, =
 torqued the bolt to 10 Nm. Waited 2 hours before filling the
 tranny. Oil =
 is still dripping. Any ideas on an another cause of that
 leak?
 Best regards.
 
 Salutations=20
 Gilles Turmel
 
 ------------------------------
 
 End of vanagon Digest - 8 Jun 2016 to 9 Jun 2016
 (#2016-188)
 ************************************************************