Rigol has really annoying , really low quality rotary switches and is the side effect of a low budget. You are not alone.
Dust can be a problem but normally it is the lack of gold plating for friction contact surfaces that is mandatory for I<2A since there is no wetting (deoxiding) current. All good switch contacts <=2A must have flash gold plated contacts, which can range from 5~25 um thick I recall. Cost reductions affect this.
When infrequently used, it oxidizes. WD40 can lubricate the cutting action and connect with surface roughness which wears down plating, but that might need to be repeated every year or so.