I use 2 stoke oil in my diesel. But thats a separate topic. I have seen that they sell a lot of additives from STP. Anyone tried those?
Personally I doubt oil additives will do much assuming you use Audi/quantum spec oil, the only exception I would consider are the new nano tech oils but that would be subject to some research first. As for fuel I would run a few tanks of Vpower/Ultimate or similar and give it a few good italian tune ups!