Preheat oven to 325 degrees.
Cream the butter.
Add sugar; cream together.
Add the vegetable oil, combine thoroughly.
Add eggs 1 at a time, ensuring each egg is incorporated before adding the next.
Add key lime juice and combine; add the lime zest.
Alternate adding flour and buttermilk.
Combine well; do not overmix.
Bake in a greased cake pan at 1 hour 15 minutes to 1 hour 30 minutes (or until toothpick comes out clean).
Remove from oven and flip the pan after 10 - 15 minutes.