Hi everyone...

Slightly bizarre question to ask for my first one but....

Just checking the oil/water etc under the bonnet of my 2005 Passat Estate 103 TDi when I noticed, on the right hand side of the engine bay in front of the windscreen washer bottle there is a green screw cap. On removing it there was a small dip stick attached to teh cap and a black, oil type fluid inside.

Can't find any pictures or references to what this is in the VW hanbooks and I've read the thing from back to front. No sign or mention of it in any of the engine bay pictures either...what is it ?