I was thinking along these lines recently while I have been searching for a replacement for the F31. Right now it seems that a petrol 3 series is more expensive than the equivalent diesel, entirely down to VRT differences. I don't see much point in paying more now to get a slightly higher residual in a few years so I have decided to buy another 330d. I wont keep it for anything like 5 years, I'd say the max I will keep it for would be a year or two at which point I might well buy a petrol car.