There are several different methods for removing oil stains on granite countertops, but they aren't all good for the surface. The mildest method is to mix baking powder and water, form it into a paste and cover the stained area. Once the paste is on, cover it with a plastic wrap and let it sit overnight. Wash it off the next day.
Another method calls for flour, dish detergent and water. This should not be as thick as a paste but more like pudding. It is applied and covered in the same way as above and washed off the next day.
If you don't mind the expense or are not in the mood to experiment, there are products that can be bought to remove grease and oil stains from granite countertops. They are normally found in home improvement stores or where countertops are sold. The product is applied to the area, but not covered. It is usually left on anywhere from 24 to 48 hours before removal.