I have an MX518 and a G400.
Layer of grime on the black grips of the MX518, the G400 is non-absorbent rubberized grip. They changed it for a reason. I wanted to mess around with my 518 now that I had another decent mouse to fall back on if I needed to.
I have lots of pads of medical wipes they swab you with before giving you a shot, they have a bit of a rough texture and are doused in 70% isopropyl, it took a few days of drying, rough wiping, drying, but it's finally all off.
Get a towel and the purest isopropyl you can find. Good luck.