If it's an E39 with the M52 engine, I had a 540i and never got less than 18mpg
E39 535i is the M62 V8, just lower displacement to the 4.4 litre version in the 540i. Always found it a rather pointless lump in that it offers Inline Six 530i performance yet 540i running costs. Although that fuel consumption is awful, it should be about the same as a 540i.