Well, that gland nut has been tightened fully home. Normally there are a few threads visible - are the others all fully home all as in the picture?. This leads me to think that possibly that valve does not have a traditional gland packing and so you won't be able to service it in the way you intended. It should still unscrew, but if it was tightened fully home in the factory, you'll struggle to undo it now.
Of the non-leaking valves you have, is there a definite resistance to rotation when you turn them or do they turn easily? A correctly set up traditional gland will usually feel a bit stiff whereas O ring sealed glands have less resistance to turning.
3 in 1 oil won't dissolve limescale. It was actually (if legend has it right) designed as an oil which would coat the exposed surfaces of a bicycle chain (and so tends to clog precision mechanisms). WD-40 isn't much good as a lubricant, but it's good for cleaning purposes and it does seem to shift limescale, which was why I suggested it.
Of the non-leaking valves you have, is there a definite resistance to rotation when you turn them or do they turn easily? A correctly set up traditional gland will usually feel a bit stiff whereas O ring sealed glands have less resistance to turning.
3 in 1 oil won't dissolve limescale. It was actually (if legend has it right) designed as an oil which would coat the exposed surfaces of a bicycle chain (and so tends to clog precision mechanisms). WD-40 isn't much good as a lubricant, but it's good for cleaning purposes and it does seem to shift limescale, which was why I suggested it.