Hi,

First- there is a "Thread kit" to repair that common problem. Even VW cary
this one.

second- There is a very good "somewhat new" seal on the market that you can
buy at any auto part, just ask for what they have for drain plug. I can send
you a picture of this one if you want.

Third- put some locktite overthere for now, i did that for a few 3000 miles
oil change when i was...... far away from home. Use some flexible oil
resistent silicone, it can be remove very easily.

Cheers, Ben
http://www3.sympatico.ca/huotx/engine1.htm