I would get some waterless hand cleaner like “Goop,” “Permatex,” or “Purple Power” and, following the instructions on the package, including trying it on a hidden area, rub a gob of the cleaner into the stain and allow it to penetrate for 10-20 minutes. Then wash the jacket as instructed on the jacket label. This should work whether it is petroleum based grease (like axle grease) or organic grease (like cooking oil or lard type grease. It’s possible you may have to do it twice. Good Luck!