Gerlacr,
About the best thing for that is kerosene. Don't use diesel and DON'T use gasoline. Kerosene is light enough that it will act as a solvent, cleaning agent, but should completely drain out of the transmission, with the lingering yuk. With the kerosene in there, you could briefly operate it (a few minutes) to circulate all bearings and gears. After that, drain the kerosene, refill with fresh cheap oil, operate for long enough to get it hot, and drain again. Then put in good oil and your good to go.
One has to wonder where the yuk came from and if any damage has been done to bearings, etc. For your sake I sure hope not.
Having said all the above, I am not a professional mechanic, but I have played one on TBN./w3tcompact/icons/laugh.gif
Nick