I hate to shoot down your guess, but nothing was oiled in the construction of the Shards of Narsil since there was no real steel used for the blades
The causes for those type of marks are either the polybag chemically breaking down onto the surface due to heat, or heat and oil chemically breaking down plastic. Or the nickel oxidizing under the clear coat. It's rare, but have seen it happen. I have also seen a type of mold grow on the surface of these parts that has a very similar pattern, but that is also rare and only happens when they are stored in dark, humid places for several years.
There is always a clear coat on anything plated, unless it is chrome. There is not a single cast zinc external part on any LOTR sword that does not have a clear coat. Nickel, brass, and gold plating all oxidize and change color, so they have to be sealed. The dull surface finish you see on the blade shards is actually the clear coat. It's a 60% shiny/40% dull mix. The nickel silver plating underneath is shiny but with a slightly dull satin sheen from a light polish with steel wool. If you strip the clear paint off, that's what you will see.
Metal Glo will polish the clear coat shiny with light rubbing, and with heavy rubbing it will strip the clear off - but it does take some work. I partially strip the clear coat down for a weathering effect, like on the aged Sting scabbard I customized last year. It does take some work to get it off though.
I should also say, I'm not too sure what United sells now is the same old Metal Glow formula. The paste in the last tube I bought a few months ago looked and smelled different. It polished OK, but I did not notice the carbon breaking down very much (it turns your cloth black), so they may have changed the mix, or changed to something completely different. Flitz polishing paste is the exact same stuff that was in the old Metal Glo, so that is another option.