Gearbox oil and grounds
You may wish to use Stabilant 22 on the contacts after cleaning them. When a small amount of Stabilant 22 is applied to the surfaces of electromechanical contacts, it enhances there conductivity_ and provides them with the reliability of a soldered joint without bonding the contacting surfaces together! It increases the reliability of contacts by anywhere from 10 to 100 fold.
You can purchase it at VW or Audi dealer parts depts, in addition to other computer/sound equipment shops.
I have found that in over 236,000 miles, Redline gear oil has helped me to extend the life of the synchros. I am still on the original 'box, but it is getting a little crunchy on the 1st to 2nd shift now if I don't take care.