The best way to clean a derailleur, pulley wheels, and cogs, I have found, is throwing them out and buy an IGH.





No, seriously, though: You can get some citric acid power, dissolve a little bit in boiling water, add cold to cool it down so it's safe to handle. Use a brush. Best degreaser ever (for non-brakes). Rinse with cold water. It dissolves fats and oils very efficiently. It is also better than vinegar for descaling your kettle. Fill the kettle with cold water, put in the citric acid powder and boil it. Rinse after.