I usually smear a high temp axle grease on the speedo cable housing that plugs into the trans lubricating the o-ring. in addition a little "right-stuff" on the flange that touches the block. the propellant (air) in the right-stuff expands and makes a kinda silicone foam seal that can be helpful as well.
additionally I usually pack a little hi-temp red grease in the speedo cable itself to aid in smoothing the rotation and reducing "needle bounce".
If you have trans
oil expanding and pushing
oil up the speedo cable , you may have a transmission heat issue , combined with overfull transmission fluid.
---Steve