First, don’t panic! Oil can leave a mark in suede or leather if left on the boot for an extended duration after it initially happens. Whilst it is not something that can be dealt with directly, there are ways of reducing stains and damage.
Talc or Corn-starch can be used on the suede or leather to pull out any excess oil. However, please do so at your own risk, and don’t be overly vigorous with the amount used! Allow the corn-starch to pull out the oil as much as possible over the period of an hour, then brush off the excess.
Any remaining can be removed with a lightly moistened cloth. Follow this by brushing down the suede – the process may need to be repeated for heavy oil stains, so repeat as necessary.