IMHO, you're better off getting something designed specifically for this application. Go to your local auto parts store and see if they sell
oil absorbing mats. You can mail order a big piece and cut it down as needed, replacing them when the backing gets worn (that's what's keeping the oil from soaking through to the carpet). My local Pep Boys used to have them under the Oil Dri brand but I haven't seen them there lately. Finish Line used to sell small mats that had a more durable backing but I don't think they make them anymore.