From a post by Dewjantim:
To make a simple, effective cable lubing device take a couple of inches of fuel line and put a clamp on the end of it. Take the cable loose and clamp the fuel line tightly to the end of the cable. Pour motor oil into the fuel line, being careful that the cable is pointed straight into the air. Let set overnight. The oil will drain into the cable and it will stay lubed for a long time.....Dew
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I would add to Dew's technique that you can probably push the oil quickly through using a dowel as a ramrod inserted into the open end of the hose.
I still have a well-cared for JC Whitney cable lube syringe that's about 15 years old. It's a standard plastic medical syringe (I think 100cc) with a clear 4 inch hose clamped to it. A rubber stopper goes over the cable end and is inserted into the hose. A hose clamp tightens it all up. Then you just pour in oil and apply gentle pressure until it drips from the other end. (Pushing the exposed cable end in and out from the bottom end expedites the flow as it draws the oil downward).
These simple but effective lubricators cannot be found anymore. I keep thinking about finding sources and then posting do-it-yourself instructions
For the RE maintenance-obsessed, all it takes is consistently, every 5000 miles, removing the cable completely, inspecting it thoroughly, pressure lubricating it with your lubricant of choice, cleaning both exposed ends, clean mounting points and lubing lever and bracketry pivot points; also making sure that it is reinstalled properly. Then every maintenance in between (1850 miles for our REs) cleaning exposed portions and re-lubing with your lubricant of choice, then carefully inspecting for wear. :-)
Matt