Alright, i will go with this one for now, Mr. 535D.
(Is it a saloon and is it an M Sport model?)
Use the 'Ulitmate diesel' (or similar) every time.
It won't make it any more powerful, or if it does it's too small an increase to notice, but it does keep the engine cleaner and some people feel the engine is fractionally smoother on it, i was undecided on that one.
I did use it all the time though in my 535d, just as i now always put Ultimate unleaded in my M3 CSL and Westfield Seight.
Get a re-map by DMS, like i said and also, like i said, let me know when you want to do it because i can get it done £150 cheaper than usual for you.
Where abouts are you in the country and are you going to attend one of our Derby or London meetings soon?
I'd love to see your car.......
6R4. The Beast!