Hydrogen peroxide (more concentrated the better) mixed with so clean detergent booster, make into a page and let sit on the skull for 1 hour, will take that crap off and leave it “bone” white…
Make sure to remove the ear canals. It can help sometimes to drill a small hole into the brain cavity from the bottom with a 1/2" wafer blade to flush everything out of there. On tough, greasy skulls, I sometimes let them soak in a room temp hydrogen peroxide/water solution for a day or two and it seems to help whiten them up without weakening the skull too much. Let us know if you find a way that works for you!