ummm somethign i've noticed reading through this, no one has mentioned nicosil.
quite a few enignes now have nicosil coated cylinders / liners. nicosil dosent like sulfer at all. run an engine with nicosil liners on povery speck fuel and it attacks the coating on the liners, once thats gone they wear like crazy. problem come is that you can't fix it you canb rebore them bit no way of reapplying the coating so you need to replace the liners or the whole block as i don't think vw use wet liners at all