Unfortunately you can't disconnect from the "easy" transmission side - you have to reach up and disconnect from the speedo side. The clip is fairly self explanatory if you can see it, but I don't remember exactly how it disconnects. I think it's just a squeeze and pull operation. Once disconnected, you can pull the cable out of it's jacket. I think just about any grease would work, I probably used just regular chassis grease or maybe white lithium. BUT....I think I'd recommend replacing it instead of going to this trouble just to grease. They are not expensive, especially if you just replace the cable and not the jacket. But I would recommend buying a quality, correct length from one of the vendors. My first try was a non-specific part from the Help section of a parts house - not a good idea.
______________
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.