When the cassette spacers grind into dust
3/4 real mineral spirits and 1/4 straight 20 Wt Mobil1. Apply when it starts to dry, wipe off before you ride. Gets a little gooey on the pulleys but wipes right off. Gets the chain really clean and I made my concoction back when GWB was still president. Works great and it's really quiet. Totally sold on synthetic oil