The speedometer cable drive is in the transmission. The transmission factory oil fill was 90 wt gear oil. Check the oil level in the transmission, if it is not overfull then the speedometer cable access hole seal is leaking.
Also the transmission breather may be plugged causing a slight pressure buildup as the oil warms that is relieving through the speedo access hole seal.
At high speed the gears will churn the oil and throw it in all directions including up against the top covers.