How do I divide Pilsbury professional cake mix I have a 10lb bag which requires 8 cups of water 8 cups of oil so how do I divide it to make a smaller batch. 

Scale it.   Weight out how many oz you need equal to one 'standard mix' (that would be 16 to 18 oz).  Then follow the WASC recipe :)     http://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe

OR just use the difections on a standard box for ingredients.   I know that they are now just 15 + a fraction ounces but using the larger amount is much better.  Takes you back to what they original were and give a better final cake.