I have found that a liquid graphite suspension is good for this, if you can find it in liquid form. Even if the oil gets depleted over time, the graphite will remain.
Some manuals I have seen for various cars have suggested using ATF, or a very thin oil that is capable of soaking down the cable.
With persistance, you can usually get in to start dripping on the tranny end, which indicates you really got it.

I usually put a wrap of electrical tape around the jacket end to form a 'well', then keep filling it until oil runs out the other end.

I once saw a tool that clamped to a cable end, had a zerk fitting, and would allow one to pump grease into a cable. Thought that was a great idea, but did not buy it at the time, and I have not seen one since. Web search came up empty. Anyone who has seen such a tool, let me know...

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1986 300E 5-Speed 240k mi.