I will say that I am not any kind of mechanical technician, but that photo of gunk and schmuttz looks much like my derailleur just before I get out the electric pressure sprayer. I try to keep the spray parallel to the spokes rather than parallel to the axle, but I know that water most likely gets into the works. I should take it apart, but instead I spray a generous dose of a foaming, lanolin based (no Petro solvents) industrial lubricant. I tried not wiping off the excess, but the excess winds up on the brake rotor. Better to wipe the chain clean, wipe the sprockets clean, anything else you can reach with a rag covered finger, wipe the excess grunge and lube off. Then relube the chain and do it again. And spray or squirt lube into the rotating cage hubs and wipe down afterwards. And don't wait so long to do it again next time. At least, that's what I tell myself.