I've just got a new fuel additive to put in my 504 Ti, lead supreme-130, which contains tetraethyl lead. It's an American product that's distributed by Millers Oil. The info i got with it claims that one 900ml bottle will treat 360 litres of fuel (25ml/10 litres petrol) and gives an octain boost of one point. It is also approved for use by the EPA.
I have to fuel the car up this arvo, so I'll add some in then and see how it goes.