This method is best used for those old, established stains that will be difficult-if not impossible-to remove. Asphalt sealers, a coal-tar product, can be found at any hardware store and will work to mask the stain. This is probably best as one of your last efforts to remove the stain as perhaps it can be done using the other, cheaper methods mentioned here.
Of curse, seal coating can only be properly done once the driveway has been cleaned as best as is possible, so be sure to utilize one or more of the other methods to ensure you've removed as much of the oil stain as possible.