I had good success with hydraulc fluid. I would clean out all the grease thoroughly by spraying cheap degreaser into cable and working it in and out and repeating many times, cleaning out sterring tube with degreaser and rag on a drill bit etc and then put on a felt seal on one end and use Dexron III or similiar in the tube. Makes a huge difference and doesnt seize up. I pulled and replaced fluid once every 2 years.
Cheers