Congrats to the Haynes people, congrats to Savage Greg -- it is still a good trick wherever it came from or from or from whoever teaches it to you.
(it was in the 1970's era Clymers Honda S90 book as well)
Key point of the trick is you can lube a cable with a significant amount of a long lasting oil with the cable still sitting in place on the bike.
If there happens to be a down & up loop in the cable run, well it gets completely filled up with oil and the oil stays there as a reservoir to keep suppling oil to the rest of the cable by osmosis through the cable weave itself.
Long lasting, the heavy oil baggie trick ...
Yes, you can use a lighter oil if you want to. But if you want that buttery smooth feel of a clutch cable floating in a supporting sea of 90-140 weight gear oil nothing else will substitute.
Fair warning though, our clutch cables continue to ooze the heavy gear oil for a long period of time and it winds up making little oil droplettes on the clutch to engine lever bracket that you have to mop up occasionally.
I don't mind, shows me it's still there and still working.
Side benefit is your little rubber bellows won't dry rot near a quick when they have an internal coating of heavy gear oil -- they stay nice and fresh and flexible for a long time.
I haven't noticed the grit you mention, but I am sure I am wiping it off ongoing as I have to clean the bike periodically. Have any of you who ride all year noticed that in the winter time all the little birds like to perch on your warm engine cases to warm their little cold feet up (must be like a emergency bird hostel to them little brown tweety birds)? Leave you little white presents on your cases, they do.
I have to wash my bike more in the winter than I ever have to do in the summertime .....