Hello and welcome to forum bossyboots.
When removing grease or oil stains from leather, the best advice is not to use water to clean up the spill. Water and oil do not mix well. Water will cause the oil to spread even more.
Try using cornstarch or talcum powder; either one will act as an absorbent to lift away the oil or grease from the leather material.
Start by using a dry cloth and blot the excess oil away. Place a small amount of either cornstarch or talcum powder on the stain (enough to cover the affected area)
and leave on for a few hours then check the progress; you may have to leave the mixture on overnight. Then using a soft dry cloth wipe the mixture away.
You can also purchase a leather stain de-greaser. You should be able to purchase this at any store that sells leather goods.
If one of these do not work you might want to take the boots to the shoe repair shop.
Let us know what worked for you.
Mrs. Clean