Originally Posted by choddo

I actually meant the derailleur itself but the cogs (the cassette) definitely don’t need oil. They will always pick up a little from the chain but they have no parts that move relative to each other. They will get slightly rusty if you leave them wet but usually running the chain backwards through a cloth if it’s being stored after going out in a downpour is enough to avoid any problems. I never oil the front chainrings or rear cogs. Oil shouldn’t really be used to avoid rust (wd40 or grease can be but soraying wd40 anywhere in the vicinity of the braking surfaces on a bike is a bad idea so I don’t do that either) but rather to lubricate metals that move against each other, e.g. the plates in the chain as it goes around a cog.

A mate taught me a neat little trick for getting those two little jockey wheels clean. Get a rag and carefully press a small bit of it against the exposed side of the rotating wheel/cog with a fingernail and rotate the pedal backwards. It will peel off any crud on there really nicely.