You need to take the mixer apart and replace the grease and the gearbox gasket.
KitchenAid uses a vegetable based grease in their mixers, the grease can break down over time, especially if you do not us the mixer often. When the grease breaks down it can leak out of the mixer.
Here is a link to the replacement grease: http://www.mendingshed.com/kagrease.html
Here is a link to the gearbox gasket:http://www.mendingshed.com/mixergasket.html
And here is a link to the service manual which will aid you in taking the mixer apart and putting it back together. http://lib.store.yahoo.net/lib/mendingshed/kitchenaidservicemanual.pdf
Thank you,
Colleen
Customer Service
MendingShed.com
800-339-9297
Colleen, April 2013