I've used WD40 before. Never really cared for it. No particular reason really. I did kind feel like it left a bit of a residue, but that might just be me being paranoid. Honestly, Brasso works best as far as cleaning just about any amount of grime from contacts, but it is pretty abrasive, so I only ever use it when it's absolutely needed. The Isopropyl and eraser methods is the overall best thing I've found though. Cheap, easy, isopropyl dries with zero residue, and it cleans pretty well with a little elbow crease thrown in.