Theres no need to strip oil out - just clean and re-oil. The stains in question are more like a brown paint so I would try paint stripper or a Graffiti remover, I usually price myself out of these jobs. I once have to clean a patio belonging to one of the McAlpine brothers after he had his balcony resealed and they never tarped the stones, I used Johnstons paint stripper which removed the resin and fullers earth to remove the oil - the fullers earth was a pain to keep out of the pool during the pressure washing ! He sent a letter to thank me for the results which arrived weeks before the cheque