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The WD in WD 40 is for "water displacement". I'd guess, in addition to mineral oil that is left behind, the solvent/carrier portion of WD40 is water-miscible components(small alcohols?) mixed with some higher carbon, volatile solvents. As such, it will degrease/dissolve greases..which is sort of the point as the lube that Shimano uses in their shifters has a history of hardening up over time. This causes shifters to get sticky or sluggish. Do a youtube search on cleaning/restoring sti shifters..you'll get plenty of hits. The mineral oil in the formula is left behind after the lighter components evaporate..displacing any water and offering some lubrication.
The nice thing about WD40 is that it works, is mostly benign chemically(from a safety standpoint and it won't harm painted finishes or plastics/rubber), and most of it evaporates. I suppose you could use mineral spirits and then follow with a light machine oil(spray).
Last edited by fishboat; 09-05-21 at 11:18 AM.