As if leather boots aren't temperamental enough to clean, oil stains certainly seem to complicate things. Leather professionals will tell you that the only way to get your leather cleaned properly is to take to them or buy expensive kits, and for large stains, they may truly be the only solution. For small stains, however, like those on boots, there's no need to spend the extra money for something that can easily be fixed at home.
Sprinkle enough cornstarch on the stain to cover it.
Rub the cornstarch over the stain briskly until heat from the friction is felt.
Repeat with fresh cornstarch until the stain is gone.
Sprinkle cornstarch onto the stain and leave it for an hour if the oil seems to have penetrated deeply and doesn't want to come out. This will draw the oil out of the leather and make it easier to clean. Repeat the rubbing process to finish getting rid of the stain.
If the stain is large and will not come out, contact a leather professional to help.
Avoid using harsh chemicals or abrasives on leather.