Go to a beauty salon supply store., such as " Sally's" if you have one around or you can order from Van Dykes some 40% peroxide and some Basic White. Mix the two together and make a paste, use rubber gloves or a small paint brush and smear this all over the skull and put the skull out in the sun ( if you can find any!), or into your wifes tanning bed ( it works, but don't let her know!!!!) It should whiten the skull alot, but may not get all of those grease stains out. All you can do is try it. Those stains are caused from the porous skull absorbing the grease and fat from the hide. Whatever you try, post some more pics so we can check out the result!!