Oil should be runny and off brown. When was last fluids service? Check service history in handbook?? Black indicates carbon deposits being put into oil. Thick black would make me a little worried. mpg - depends very much on how you drive .... but .... https://www.carkeys.co.uk/review-archive/chrysler-grand-voyager-2-8-crd-executive-xs say about .... the best the trip computer showed was 29.1mpg (real life driving by the looks of it) http://www.parkers.co.uk/cars/reviews/facts-and-figures/chrysler/grand-voyager/estate-2001/36929/ say about ........ 33mpg (probably the hypothetical value that all companies use) Feels like you should have it back to the seller and get him to give it a once over .... Service? Sale of goods - fit for purpose.... etc etc etc etc Penguin