Audi has decided that some of its S-cars are best served by diesel power after all. Is it right?Why we ranit: To see if diesel power really can excite as much as petrol in a sporting four-doorMonth 4 -Month 3 -Month 2 -Month 1-Prices and specsLife with an Audi S5 Sportback: Month 4Running this fast, red, surprisingly attention-grabbing Audi S5 for the past few months has been bitter