Its leaking out the vent on the top the gear box...mine does the same thing runs down the sides and gets all over the pulley. I have alot of hills so it seems to make it worse...at the end of the year i just degrease it and store it away for the winter. Ive been doing this for 5 years and no problems.
You probably have too much oil in it for it to be blowing out the the vent. Try keeping the oil down a hair.