Sounds like wherever you're hooked for vacuum is not releasing immediately when you shut down. A quick confirm would be to disconnect the vacuum line from wherever it is. If the drip stops, the unit is working and you need to find a proper vacuum connection. If the drip continues, it is a unit issue. Either out of adjustment or faulty.
I've used both oils and currently use 90 gear oil. The only time I've had a real issue is an emergency use of 30wt. It did a good job of lubing up the chrome on the rear wheel at the lowest setting.
