Please don't blame me for (endlessly-) repeating myself, but I would suggest to spray sparingly some Ballistol gun oil in the rims where the dark slides are stuck.
This gun oil easily penetrates into the tiny surface structure and solves/dislodge what causes the sticking (ask the hunters).
Ballistol isn't that greasy (mineral oil), is very fine, drys without sticking, doesn't gum by aging and can be easily cleaned away with a little lighter fuel on a Q-tip.
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