Sludge is a fluid only available in Where's My Perry?. It can fill up a Doofenshmirtz Gnome which takes five fills. If it goes to Perry's tube, you fail the level and must start over again. If lava touches oil, it will turn into lava. If it touches a regular gnome, it will be destroyed, making a splatting noise. It is very similar to mud, but doesn't turn into dirt.