Sealant for Thermo Housing

[The Corbs]

I'm in the process of replacing a leaking housing in the old Jetta VR6.

I have all the replacement bits from ECS Tuning (plastic is 
cheap!!),including all the O rings.

However, since I've had to remove the whole front end to access this area, 
I'd really like to ensure that it stays sealed for the future.

What if anything do you use in conjunction with the O rings?

Thanks, sorry for the thin Audi content.

John Corbs