Also, check that the transmission vent tube is not plugged. As the oil in the tranny heats up it expands and creates pressure. If the vent tube is plugged the pressure can push the oil up the Speedo cable.
You should also check that you do not have too much oil in your tranny.
It would be a good idea to disconnect both ends of the Speedo cable and flush the excess oil out with break parts cleaner. Sooner or later the excess oil will gum up the cable.