Remove the cover and look around real carefully. Most likely the low gear housing is cracked. If not the best sealant is Loctite 518 with the primer being used as directed. Other sealants such as silicone will just get squeezed out. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Gilles H Turmel
Sent: Saturday, April 16, 2016 5:11 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Loosing transmission oil

Hi, I have an 89 Westy Syncro and loosing oil from the reverse gear idler cover. I’ve emptied the thanny and redo the joint with a gasket maker, torque the bolt to 10 Nm. After 1 month of running oil leak again from the same place. I have Total Transmission Gear 7 (80W-90) syntec oil in the tranny. Is there a gasket maker I should be using?
Best regards.