Oil in general may be bad but I think there is something particular to castor oil that is good in this regard. "Rubber mask grease paint" is especially made for latex masks because it doesn't break it down in the same way, and every thing I've seen says the only real diff is the castor oil. And if you want to use regular greasepaints all you have to do is coat the mask lightly with a castor oil sealer. Check out
www.screamteam.com for tips on stuff like that, I know they sell the sealer. I got a werewolf latex appliance for halloween there about 4 years ago and sealed it with their castor oil sealer and it's still fine despite the overcoat of grease paint (except for the natural wear and tear of gluing it on and pulling it back off).